2014 sept - mechanics reg review.pdf
TRANSCRIPT
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Rowel Allan Rocaberte, REE, ECE
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R
Beam
- A slender bar that carries transverse loading, that is, the applied forces are perpendicular to the bar
- Horizontal structural element that is capable of withstanding load primarily by resisting bending
Cantilever Beam
- Anchored at only one end
Overhanging Beam
- Asymmetrical placing of supports
In a beam, the internal force system consists of a shear force and a bending
moment acting on the cross section of the bar.
The internal forces give rise to two kinds of stresses on a transverse section of a beam: (1) axial stress that is caused by the bending moment and (2) shear stress due to the shear force.
R. Rocaberte || MECH 313 © 1st Sem 2013-2014
No need to discuss derivation. Dati naman hindi na.
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R
Beams are classified according to their supports.
Fixed Beam
Simply Supported Beam
R. Rocaberte || MECH 313 © 1st Sem 2013-2014
. .
P
P
No need to discuss derivation. Dati naman hindi na.
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R
Beams are classified according to their supports.
Cantilever Beam
Overhanging Beam
R. Rocaberte || MECH 313 © 1st Sem 2013-2014
. .
ω
R M
ω P
R1 R2
No need to discuss derivation. Dati naman hindi na.
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R
Loading Principles:
Uniformly Distributed Load Effective Resultant &
Moment Arm
R. Rocaberte || MECH 313 © 1st Sem 2013-2014
ω
L
R = ωL
L/2
No need to discuss derivation. Dati naman hindi na.
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R
Loading Principles:
Uniformly Varying Load Effective Resultant &
Moment Arm
R. Rocaberte || MECH 313 © 1st Sem 2013-2014
ω
L
R = ½ ωL
2/3 L
No need to discuss derivation. Dati naman hindi na.
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R
Loading Principles:
Combination of Distributed Effective Resultant &
and Uniformly Varying Loads Moment Arm
R. Rocaberte || MECH 313 © 1st Sem 2013-2014
ω2
L
R2 = ½ (ω2-ω1)L
2/3 L
ω1
ω2 - ω1
L/2
R1 = ω1L
No need to discuss derivation. Dati naman hindi na.
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R
Shear and Moment Diagrams
- Analytical tools used in conjunction with structural analysis to help perform structural design by determining the value of shear force and bending moment at a given point of an element
- Convenient visual references to the internal forces in a beam, in particular, they identify the maximum values of τ and M.
R. Rocaberte || MECH 313 © 1st Sem 2013-2014
Read also the slides on Beams by M. S. Sivakumar from Indian Institute of Technology Madras as
supplement
No need to discuss derivation. Dati naman hindi na.
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R
Shear and Moment Diagrams
Sign conventions
R. Rocaberte || MECH 313 © 1st Sem 2013-2014
External Loads
Shear Force
Bending Moment
Positive Negative
ω P M
ω P
M
V
V
V
V
M M M M
No need to discuss derivation. Dati naman hindi na.
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55. Which of the following support or connection has two unknowns?
A. Roller Support C. Short Link
B. Rocker Support D. Hinge Support
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56. A simple beam, 10 m long carries a concentrated load of 500 kN at the midspan. What is the maximum moment of the beam in kN-m?
A. 250
B. 500
C. 1000
D. 1250
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57. Determine the maximum moment of the beam loaded as shown in kN-m.
A. 125
B. 175
C. 875
D. 1250
5m 10m
200 kN
R1
R2
5m
100 kN
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58. It occurs when a member carries a load perpendicular to its long axis while being supported in a stable manner.
A. Direct shear C. Torsional shear
B. Bending stress D. Torsional stress
Bending Stress a.k.a Moment Stress
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RADIUS OF GYRATION
59. What is the radius of gyration of a 5-inch square with respect to its centroidal axis?
A. 1.44 in C. 10.39 in
B. 0.866 in D. 0.5 in square
ak =
12
4
2
square
aI 12k = =A a
5k = = 1.44 in
12
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SECTION MODULUS
60. What is the section modulus of a 5-inch square with respect to its centroidal axis?
A. 20.83 cm3 C. 25.7 cm3
B. 49.5 cm3 D. 57.2 cm3
4
squaresquare
3
square
33
square
Ise mo =
y
aI 12z = =ay
2
az =
6
5z = = 20.83 in
6
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61. What is the formula of the radius of gyration, k, of a circular cross-section with diameter D?
A. B.
C. D.
1k D
18
1k D
12
1k D
4
1k D
12
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62. What is the formula of the section modulus, z, of a rectangle?
A. B.
C. D.
3bhZ
6
2bhZ
36
3bhZ
36
2bhZ
6
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63. What is the moment of inertia of a cylinder of radius 5 m and mass of 5 kg?
A. 62.5 kg-m2 B. 80 kg-m2
C. 72.5 kg-m2 D. 120 kg-m2
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- oldest branch of Physics.
- study of the bodies and systems and the forces acting on them.
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1. Which of the following refers to the branch of mechanics dealing with the motion of bodies?
A. Kinematics
B. Kinetics
C. Dynamics
D. Statics
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Force
• The scientific definition for force is simply a push or a pull. For example, when you do homework you exert a force on your pen or pencil because you push and pull it across the paper.
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Net Force
• The sum of the forces is called the net force. In this case the net force is an unbalanced force. An unbalanced force is a force that changes an object’s motion or causes it to accelerate. The arrows show different forces and their direction, the wider the arrow the stronger the force.
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Force and Force Systems
• Collinear Forces are forces that act on the same line of action
• Parallel Forces are forces that are of the same angle to one another. Couple is a pair of parallel forces of the same magnitude but opposite in direction.
• Frictional Force is a force that always acts in opposite direction to the applied force.
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Force and Force Systems
• Coplanar Forces are forces lying on the same plane.
• Concurrent Forces are forces that meet in one common point.
• Non-concurrent Forces are forces that do not meet in one common point.
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A quantity that has magnitude (and unit) only.
E.g. speed, distance, volume, current & etc.
A quantity that has magnitude, (unit) and direction. E.g. force gravity, displacement, acceleration, momentum, etc.
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SCALARS VECTORS
Mass A load has a mass of 5kg.
Weight / Force A force of 15 N acts on a body in an upward direction.
Distance The train has traveled a distance of 80 km.
Displacement An airplane flies a distance of 100km in an easternly direction.
Speed Velocity A car moves 60km/hr, 35⁰ east of north.
Time The car has reached its destination after 1 hr.
Acceleration
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Addition: A + B = B + A Subtraction: A – B Multiplication: Dot Product: A · B Cross Product: A x B
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The sum of two or more vectors is represented by a single vector called resultant.
This resultant vector may be found by using:
– Graphical Method
– Pythagorean Theorem
– Component method
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CABLES
Used in numerous engineering applications (suspension bridges, power transmission lines, cable supporting heavy trolleys or telephone lines)
Incapable of developing internal forces other than TENSION
TWO TYPES:
1. Parabolic
2. Catenary
Front slide photo taken from http://catenary-project.wikispaces.com/Golden+Gate+Bridge
MECH 312 RROCABERTE © 2013
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PARABOLIC CABLES: Symmetric
- Loading is distributed uniformly along the horizontal
L = span; distance between supports (m, ft)
d = sag ; maximum vertical displacement (m, ft)
S = total length of the cable (m, ft) Photo taken from http://www.solutioninn.com/engineering/civil-engineering/statics/the-cable-is-subjected-to-the-parabolic-loading-w-w0-18722-2xa
L
d S
5
6
3
42
7
256
5
32
3
8
L
d
L
d
L
dLS
ω
MECH 312 RROCABERTE © 2013
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PARABOLIC CABLES: Symmetric
ω = load per horizontal length (N/m, lb/ft)
T = tension at the supports (N, lb)
H = tension at the lowest point (N, lb)
S/2
T
H
Photo taken from http://www.solutioninn.com/engineering/civil-engineering/statics/the-cable-is-subjected-to-the-parabolic-loading-w-w0-18722-2xa
L/2
2
2
2
1
LωHT
ω
2
Lω
FBD:
T
H
2
Lω
MECH 312 RROCABERTE © 2013
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PARABOLIC CABLES: Symmetric
Photo taken from http://www.solutioninn.com/engineering/civil-engineering/statics/the-cable-is-subjected-to-the-parabolic-loading-w-w0-18722-2xa
d
LωH
8
2
042
)(0
dHLLω
cwM AT
H
L/2
2
Lω
d
MECH 312 RROCABERTE © 2013
S/2
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PARABOLIC CABLES: Unsymmetric
- x and y axes are placed on the lowest point
- analyzed like two separate symmetric cables
Photo taken from http://www.mathalino.com/forum/strength-materials/parabolic-cable
O
L
dA
dB
2
LA2
LB
MECH 312 RROCABERTE © 2013
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2. A pipeline crossing a river is suspended from a steel cable stretched between two posts 100 m apart. The weight of the pipe is 14 kg/m while the cable weighs 1 kg/m assumed to be uniformly distributed horizontally. If the allowed sag is 2 m, determine the tension of the cable at the post.
A. 9047.28 kg B. 9404.95 kg
C. 9545.88 kg D. 9245.37 kg
L = 100m; w = 15kg/m; d = 2m H = wL2/8d H = (15)(100)2 / 8(2) = 9375 kg T2 = H2 + [(1/2)wL]2
T = sqrt [93752 + (1/2 x 15 x 100)2 ] = 9404.95 kg
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3. A certain cable is suspended between two supports at the same elevation and 500 ft apart. The load is 500 lbs per horizontal foot including the weight of the cable. The sag of the cable is 30 ft. Calculate the total length of the cable.
A. 503.21 ft B. 504.76 ft
C. 505.12 ft D. 506.03 ft
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The Catenary Curve MECH 312 RROCABERTE © 2013
Gateway Arch in St. Louis, Missouri [1]
Spider’s web [2]
Hanging chain [3]
Lace [5]
Freely-hanging transmission lines [4]
a
xay cosh
Dance Step Exercise:
Hawi – Sine (wave) – Wax ng Hair)
Galileo claimed that the curve of a chain hanging under gravity should be a parabola. In 1669, his claim was proved to be wrong.
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Galileo
• called the father of modern science precisely because he initiated the comparison between theory and experiment
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“Big Bang is the Day without Yesterday” - Msgr. George Lemaitre
• Lemaitre told Einstein, “Your mathematics is correct, but your physics is abominable!”
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- Loading is distributed uniformly along the length of the cable
L = span (m, ft)
y = sag(m, ft)
S = total length of the cable (m, ft)
CATENARY CABLES: Symmetric
Photo taken from http://lecture.civilengineeringx.com/structural-analysis/structural-steel/suspension-bridge-analysis/
L
y
S
T T
ω
MECH 312 RROCABERTE © 2013
A B
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SAO = length of the cable from A to O (m, ft)
x = horizontal span from A to O (m, ft)
H = tension at the lowest point (N, lb)
ω = weight per unit length of the cable (N/m, lb/ft)
CATENARY CABLES: Symmetric
Photo taken from http://lecture.civilengineeringx.com/structural-analysis/structural-steel/suspension-bridge-analysis/
y SAO
T
ω
MECH 312 RROCABERTE © 2013
A
x
H O
H
xω
ω
HSAO sinh
1cosh
H
xω
ω
Hy
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T = tension at the supports (N, lb)
Recall equation of a catenary
CATENARY CABLES: Symmetric
Photo taken from http://lecture.civilengineeringx.com/structural-analysis/structural-steel/suspension-bridge-analysis/
T
MECH 312 RROCABERTE © 2013
x
H
22
AOSωHT
AOSω
FBD:
T
H
AOSω
yωHT
a
xay cosh H
xωHT cosh
y
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Note:
For symmetric catenary cables,
If , you may solve catenary cable problem as
a parabolic cable.
(A small sag-to-span ratio means that the cable is tight, and the uniform distribution of weight along the cable is not very different from the same load intensity distributed uniformly along the horizontal.)
CATENARY CABLES: Symmetric MECH 312 RROCABERTE © 2013
Ly10
1
AOSS
xL
2
2
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Similar to parabolic (unsymmetric):
- x and y axes are placed on the lowest point
- analyzed like two separate symmetric cables
CATENARY CABLES: Unsymmetric
Photo taken from http://lecture.civilengineeringx.com/structural-analysis/structural-steel/suspension-bridge-analysis/
L
TB
TA
MECH 312 RROCABERTE © 2013
A
B
x
y
O SAO
SOB
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4. A cable which has a mass of 0.6 kg/m and is 240 m long is to be suspended with a sag of 24 m. Determine the tension at midlength, maximum tension and maximum span.
A. 233.55 m
B. 240 m
C. 265.216 m
D. 190.98 m
MECH 312 RROCABERTE © 2013
22
AOSωHT
yωHT
H = 1696.39 T = 1837.75
H
xωHT cosh
x = 116.78 L = 2x = 233.55 m
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Speed
The average speed of an object is equal to the total distance traveled in a given unit of time. It is a scalar quantity because it has no direction .
t
dvs
Speed and velocity are often used interchangeably in ordinary conversations. In physics, there is a clear distinction between these two concepts.
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Velocity We define velocity as the time rate of change of
position.
The velocity of an object moving along a straight path is equal to the slope of the d-against-t graph. When the graph is a straight line, the velocity is constant.
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Velocity The average velocity of an object is the total
displacement divided by the elapsed time.
t
dvave
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Velocity
Instantaneous velocity is the velocity of the object at a particular instant. Its magnitude is equal to the slope of the line tangent to the point corresponding to the given time t.
iedt
dsv ˆ
vectorgentunitei tanˆ
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Acceleration We define acceleration as the time rate of
change of velocity.
average instantaneous
acceleration acceleration
t
vaave
dt
vda
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Rectilinear Translation Rectilinear Translation
A type of motion in which a body moves in a straight line or is moving in the direction parallel to its displacement.
Uniform Motion
A motion with constant speed or velocity [a = 0]
Uniformly Accelerated Motion
A motion with constant change in velocity or of uniform acceleration [a is (+) is accelerating or speeding up; a is (-) if decelerating or slowing down]
D1.
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Kinematic Differential Equations of Motion
s = displacement
v = velocity
a = acceleration
t = time
dt
dsv )1(
dt
dva )2(
dvvdsa )3(a
dv
v
ds
andfrom
),2()1(
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Rectilinear Translation For uniformly accelerated motion along a
straight horizontal path:
atvv
asvv
attvs
o
o
o
2
2
1
22
2
s = displacement vo = initial velocity v = final velocity a = acceleration t = elapsed time Uniform velocity: a = 0, v = vo
Starting from rest: vo = 0 Stop at a point: v = 0
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5. From the speed of 100 kph, a car decelerates at the rate of 15 m/min/sec along a straight road. Which of the following gives the distance travelled at the end of 40 sec.
A. 3800 m
B. 911.112 m
C. 91.111 m
D. 455.56 m
V0 = 100kph x 1000
/3600 = 27.7778 m/s
a = -15/60 mps2 = -0.25
mps2
s = 27.7778(40) + 0.5(-
0.25)(40)2 = 911.112 m
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Practice Problem: An airplane lands on a carrier deck at 200 mi/h and is brought to a stop uniformly, by an arresting device, in 50 ft. Find the time required to stop.
• A. 0.34 sec C. 0.46 sec
• B. 0.21 sec D. 0.86 sec
There’s no other way but up!
0.34 sec
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Practice Problem: A sports car starting from rest can attain a speed of 60 mi/hr in 8 seconds. A runner can do a 100-yard dash in 9.8 seconds. Assume that the runner is moving with uniform speed and that the car starts at the instant he passes it. How far will both travel until the car overtakes the runner?
• A. 180.3 ft C. 200.3 ft
• B. 190.3 ft D. 170.3 ft
There’s no other way but up!
170.3 ft
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Practice Problem: What average net thrust must a 17-ton airplane have in order to reach an altitude of 5,000 ft and a speed of 600 mi/hr at an airline distance of 10 mi from its starting point?
• A. 11,000 lb C. 14,000 lb
• B. 13,000 lb D. 12,000 lb
There’s no other way but up!
11,000 lb
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6. An airplane acquires a take-off velocity of 150 mph on a 2-mile runway. If the plane started from rest and the acceleration remains constant, what is the time required to reach take-off speed?
A. 40 s B. 45 s
C. 58 s D. 96 s
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Free Falling Body - straight line free fall
- motion in vertical direction
ga
For free fall: where g = acceleration due to gravity = 9.81 m/s2 = 32.2 ft/s2
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Free Falling Body Same formulas for rectilinear motion
gtvv
gyvv
gttvy
o
o
o
2
2
1
22
2
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Free Falling Body
from A to B: y (+) v (+) g (-) from B to C: y (+) v (-) g (-) from C to D: y (-) v (-) g (-)
B (highest point)
A C (reference point)
D
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Free Falling Body
y (+) above the reference point
y (-) below the reference point
v (+) upward motion
v (-) downward motion
g (-) always negative
B (highest point)
A
D
C (reference point)
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7. A stone is thrown down from the top of the cliff 150 m high with an initial speed of 30 m/sec. How long will it take to reach the bottom?
A. 3 sec B. 2.5 sec
C. 4.56 sec D. 3.26 sec
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8. A ball is thrown vertically upward from the ground and a student gazing out of a window sees it moving upward pass him at 5 m/s. The window is 10 m above the ground. How high does the ball go above the ground?
A. 1.276 m
B. 11. 276 m
C. 5.276 m
D. 15.276 m
MECH 312 RROCABERTE © 2013
10
222
hH
ghvv o
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Practice Problem: A stone is dropped down a well and 5 seconds later the sound of the splash is heard. If the velocity of sound is 1120 ft/s, what is the depth of the well? Ans. 353.31 ft
MECH 312 RROCABERTE © 2013
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Projectile Motions
• A projectile is an object or body thrown with an initial velocity and whose motion is influenced by the pull of gravity
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Projectile Motions
• Projectile motion is the motion of a body (the projectile) with a constant acceleration.
• It is principally determined by two types of motion: vertical and horizontal motions.
• Trajectory is the path of a projectile. Projectiles follow a parabolic path.
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222
2
cos2
cos
cos2
1
θvxavv
θvtavv
tθvattvx
oxoxx
oxoxx
oox
Flight of Projectile
θ
xmax
vo
voy
vox
y = 0 y = 0
vy = 0
ymax
gyθvyavv
gtθvtavv
gttθvtatvy
oyoyy
oyoyy
oyoy
2sin2
sin
2
1sin
2
1
222
22
θvv
θvv
ga
a
motionprojectileFor
ooy
oox
y
x
sin
cos
0
,
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Range of a Projectile
g
θvR
g
θθv
g
θvθv
tθvtvxR
g
θvt
gttθv
yRangeforgttvy
o
ooo
oox
o
o
o
2sin
cossin2sin2cos
cos
sin2
2
1sin0
0,;2
1
2
2
max
2
2
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Height of a Projectile
g
θvH
gHθv
vHeighttheat
gHvv
o
o
fy
oyfy
2
sin
2sin0
0,max
2
22
22
22
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If final position is not the same as the original level, we have values for both x and y
θv
gxθxy
θv
gx
θ
θxy
θv
xg
θv
xθvy
θv
xtlet
gttθvy
tθvx
o
o
oo
o
o
o
o
22
2
22
2
2
2
cos2tan
cos2
1
cos
sin
cos2
1
cossin
cos
2
1sin
cos
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Guide Question
9. A shot is fired at an angle of 45˚ with the horizontal and a velocity of 300 fps. Calculate the range of the projectile.
A. 3500 yd
B. 2800 yd
C. 1471 yd
D. 932 yd
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Guide Question
10. A projectile leaves at a velocity of 50 m/s at an angle of 30˚ with the horizontal. Find the maximum height that it could reach in meter.
A. 41.26
B. 28.46
C. 31.86
D. 51.26
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11. A projectile is fired from a cliff 300 m high with an initial velocity of 400 m/s. If the firing angle is 30° from the horizontal, compute the horizontal range of the projectile.
A. 15.74 km B. 14.54 km
C. 12.31 km D. 20.43 km
y=xtanθ-gx2/2(vocosθ)2
-300=(tan30)x-9.81/2(400cos30)2 x2
x=14.63km
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at = r α
at = r ω2
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12. The normal acceleration of a particle on the rim of a pulley 10 ft in diameter is constant at 1200 fps2. Which of the following gives the speed of the pulley in rpm.
a. 77.4597 b. 15.4919
c. 147.9371 d. 73.9686
an = v2/r, 1200 = v2/5,
v = 77.4597 fps
v = rω, 77.4597 = 5ω, ω
= 15.4919
rad/sec*60/2Π =
147.9371 rpm
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13. A discus thrower moves the discus in a circle of radius 80.0 cm. At a certain instant, the thrower is spinning at an angular speed of 10.0 rad/s and the angular speed is increasing at 50.0 rad/s2. At this instant, find the tangential and centripetal components of the acceleration of the discus and the magnitude of the acceleration. a. 89.1 m/s2 b. 89.2 m/s2 c. 89.3 m/s2 d. 89.4 m/s2
2 2
tan
2 2 2
2 2 2
tan
(0.800 )(50.0 / ) 40.0 /
(10.0 / ) (0.800 ) 80.0 /
89.4 /
rad
rad
a r m rad s m s
a r rad s m m s
a a a m s
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Newton’s Laws of Motion
- The basis for extending the kinetics of a particle to a body composed of a system of particles.
“particle” – denotes an object of point size
“body” – denotes a system of particles which form an object of appreciable size
Any rigid body, regardless of its size, may be considered to be a particle if all of its parts move in identical parallel paths.
MECH 312 RROCABERTE © 2013
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Newton’s Laws of Motion
First Law (Law of Inertia)
A body acted on by no net force moves with constant velocity (which may be zero) and zero acceleration.
Second Law (Law of Acceleration)
If a net external force acts on a body, the body accelerates. The direction of acceleration is the same as the direction of the net force.
Third Law (Law of Interaction)
If body A exerts a force on body B, then body B exerts a force on body A. These two forces are equal in magnitude but are opposite in direction; a.k.a. “Law of Action and Reaction”
MECH 312 RROCABERTE © 2013
aFFne t m
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Newton’s Laws of Motion for a Particle
Inertia is the resistance a body offers to a change in its motion.
The reference frame or set of axes in which Newton’s laws are valid is one having a fixed origin and fixed directions of the axes. It is called an inertial, Newtonian, Galilean frame of reference.
Absolute motion - motion with respect to an inertial frame.
MECH 312 RROCABERTE © 2013
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Every particle in the universe attracts every other particle with a force which is directly
proportional to the product of the masses of two particles and inversely proportional to the square of the distance between the centers of
the masses.
Newton’s Law of Universal Gravitation
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Fg = gravitational force
G = 6.673 x 10-11 m3/kg-s2 (CONST 39)*
m1 & m2 = masses of the particles (kg)
r = center-to-center distance (m)
1 2
2g
m mF G
r
Gravitational Force
*Do not confuse G with acceleration g due to gravity.
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Effective Force on a Particle
The effective force on a particle is defined as the resultant force on a particle, e.g. R or ma.
Inertia force the reaction caused by a resultant force
force numerically equal to ma but directed oppositely to the acceleration
Dynamic equilibrium: when inertia force is considered to act on a particle together with the resultant force, the particle is in a state of equilibrium.
MECH 312 RROCABERTE © 2013
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D’Alembert’s Principle
The resultant of the external forces applied to a body (rigid or nonrigid) composed of a system of particles is equal to the vector summation of the of the effective forces acting on all particles.
The impressed forces acting on any body are in dynamic equilibrium with the inertia forces of the particles of the body.
MECH 312 RROCABERTE © 2013
P2
P1
W
332211
21
amamamR
PPWR
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Newton’s Laws of Motion for a Particle
Newton’s second Law for any particle may now be expressed as
If the effective forces miai are reversed and considered to act on each respective particle of the system that the system will be in a state of balance known as dynamic equilibrium.
MECH 312 RROCABERTE © 2013
n
i
ii
n
i
i m11
aF
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Guide Question
14. Riders in a bus are pushed forward during a sudden stop. Which law of motion provides an explanation?
A. Law of Inertia
B. Law of Interaction
C. Law of Universal Gravitation
D. Hooke’s Law
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Guide Question
15. The force required to maintain an object at a constant speed in free space is equal to ____.
A. The weight of the object
B. The mass of the object
C. Zero
D. The force required to stop it
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Guide Question
16. A rock is dropped out of the window of a moving car. At the same time a ball is dropped from the rest of the same height. Neglecting air resistance, which will reach the ground first?
A. Rock will hit the ground first.
B. Ball will hit the ground first.
C. Both will hit at the same time.
D. Neither will hit the ground.
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Translation: Analysis as a Particle
The fundamental equation of kinetics for a particle
MECH 312 RROCABERTE © 2013
aaR
FR
g
Wm
0
:
yy
xx
aF
aF
g
W
g
W
motionrrectilineaFor
tt
nn
aF
aF
g
W
r
v
g
W
g
W
motionrcurvilineaFor
2
:
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17. Two blocks A and B are released from rest on a 30ᴼ incline when they are 60 ft apart. The coefficient of friction under the upper block A is 0.20 and that under the lower block B is 0.40. Compute the acceleration of block A as it moves down the incline. A. 11.52 m/s2 B. 12.52 m/s2m C. 14.52 m/s2 D. 10.52 m/s2
MECH 312 RROCABERTE © 2013
μA = 0.2
μB = 0.4 30o
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Practice Problem: Two blocks A and B are released from rest on a 30ᴼ incline when they are 60 ft apart. The coefficient of friction under the upper block A is 0.20 and that under the lower block B is 0.40. Compute the acceleration of block B as it moves down the incline. Ans. 4.946 m/s2
MECH 312 RROCABERTE © 2013
μA = 0.2
μB = 0.4 30o
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Practice Problem: Two blocks A and B are released from rest on a 30ᴼ incline when they are 60 ft apart. The coefficient of friction under the upper block A is 0.20 and that under the lower block B is 0.40. Compute the elapsed time until the blocks touch. Ans. 4.56 sec
MECH 312 RROCABERTE © 2013
μA = 0.2
μB = 0.4 30o
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Centripetal Force
A force that makes a body follow a curved path: it is always directed orthogonal to the velocity of the body.
A force which keeps a body moving with a uniform speed along a circular path and is directed along the radius towards the center.
from Latin centrum meaning "center" and petere, meaning "to seek"
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Centrifugal Force
Represents the effects of inertia that arise in connection with rotation and which are experienced as an outward force away from the center of rotation.
from Latin centrum, meaning "center", and fugere, meaning "to flee"
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Centrifugal / Centripetal Force
FC = centrifugal/centripetal force
m = mass
v = velocity
r = radius
2
C
mVF
r
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Guide Question
18. A 50,000 N car traveling with a speed of 150 km/hr rounds a curve whose radius is 150 m. Find the centrifugal force.
A. 65 kN
B. 38 kN
C. 70 kN
D. 59 kN
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Banking of Highway Curves (Case I)
Car is rotating (circular) radius
of curvature r with velocity v.
MECH 312 RROCABERTE © 2013
N
W
Nμr
v
g
W
2
gr
vμ
2
W v2 g r
Fc=
f=μN
N
W
NW
Fc f
r
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Banking of Highway Curves (Case II)
When v = vrated = 0 (car is not moving), upward frictional force is introduced to prevent skidding.
MECH 312 RROCABERTE © 2013
θ N
W
N W θ
N
Nμ
N
fθ tan
θμ tan
f=μN
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Banking of Highway Curves (Case III)
When v = vrated, no tendency to slip up or down the road (ideal banking of curve)
MECH 312 RROCABERTE © 2013
θ N
W
W v2 g r
Fc=
N W
Fc
θ
W
r
v
g
W
W
Fθ c
2
tan
gr
vθ
2
tan θ = ideal angle of banking
v = rated speed of the curve
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Banking of Highway Curves (Case IV)
When v > vrated, downward frictional force f is introduced to prevent skidding.
MECH 312 RROCABERTE © 2013
θ R
W W v2 g r
Fc=
R W
Fc
W
r
v
g
W
W
Fθ c
2
tan Φ
N
θ φ
N
f
θ+φ
gr
vθ
2
tan Φ
Φtan
..
μ
fcμ
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Banking of Highway Curves (Case V)
When v < vrated, upward frictional force f is introduced to prevent skidding.
MECH 312 RROCABERTE © 2013
θ
R
W W v2 g r
Fc= R W
Fc
W
r
v
g
W
W
Fθ c
2
tan Φ
N θ
φ
N
f θ-φ
gr
vθ
2
tan Φ
Φtan
..
μ
fcμ
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19. A highway curve has a super elevation of 7 degrees. What is the radius of curvature of the curve such that there will be no lateral pressure between the tires and the roadway at a speed of 40 mph? A. 670 ft B. 770 ft C. 870 ft D. 970 ft
MECH 312 RROCABERTE © 2013
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Example 5 20. The rated speed of a highway of 200 ft radius is 30 mph. If the coefficient of kinetic friction between the tires and the road is 0.60, what is the maximum speed at which a car can round the curve without skidding? A. 57.35 mph B. 102.34 mph C. 78.33 mph D. 23.83 mph
MECH 312 RROCABERTE © 2013
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21. Find the angle of banking for a highway curve of 300 ft radius designed to accommodate cars travelling at 100 mph, if the coefficient of friction between the tires and the road is 0.6. What is the rated speed of the curve? A. 51.08 mph B. 45.334 mph C. 67.21 mph D. 55.9 mph
MECH 312 RROCABERTE © 2013
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Test Yourself 1. vrated is used in what particular formula? 2. Formula for ideal banking of curves 3. Formula used when there is no frictional force
between the tires and the ground (no lateral pressure, no side thrust, etc.)
4. When a car is moving at its maximum, what is the formula?
5. When you consider the coefficient of friction between the tires and the road, what angle can we get? θ or Ф?
6. I want to know the coefficient of friction between my car’s tires and the ground. I parked my car on an inclined surface, θ from the horizontal. How do I determine for μ?
MECH 312 RROCABERTE © 2013
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Answers to Test Yourself 1.tan θ = v2/gr 2.tan θ = v2/gr 3.tan θ = v2/gr 4.tan (θ + Ф) = v2/gr 5.Ф if car is moving not equal to the rated speed θ if car is not moving at all 6. μ = tan θ
MECH 312 RROCABERTE © 2013
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Test Yourself Practice Problem: Determine the angle of super elevation for a highway curve of 600 ft radius so that there will be no side thrust for a speed of 45 mph. Ans. 12. 71°
MECH 312 RROCABERTE © 2013
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UNIT 4
Rowel Allan Rocaberte, REE, ECE [email protected]
+63926 – 740 – 1530
MECH 321: DYNAMICS OF RIGID BODIES (Summer 2013)
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Angular Motion. Fixed Axis Rotation.
Fixed-axis rotation is defined as that motion of a rigid body in which the particles move in circular paths with their centers on a fixed straight line that is called the axis of rotation.
MECH 312 RROCABERTE © 2013
r
r
ωo
ωf
α θ = angular displacement, rad
ω0 = initial angular velocity, rad/s
ωf = final angular velocity, rad/s
α = angular acceleration, rad/s2
Uniform velocity:
α = 0, ωf = ω0
Starting from rest:
ω0 = 0
Stop at a point:
ωf = 0
θ
2
2
dt
θd
dt
ωdα
dt
θdω
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Angular Motion. Fixed Axis Rotation.
Kinematic Differential Equations of Rotation
MECH 312 RROCABERTE © 2013
r ωo
ωf
α
θ
αra
dt
ωdr
dt
dv
ωrv
dt
θdr
st
ds
θrs
a vo
vf
s
2
22
ωra
r
ωr
r
va
αra
dt
dva
N
N
T
T
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tαωω
αθωω
tαtωθ
of
of
o
2
2
1
22
2
Rotation with constant angular acceleration
For uniformly accelerated motion along a circular path:
atvv
asvv
attvs
of
of
o
2
2
1
22
2
αra
ωrv
θrs
Rectilinear Motion Rotation (Related by)
MECH 312 RROCABERTE © 2013
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22. A flywheel of radius 14 inches is rotating at the rate of 1000 rpm. How fast does a point on the rim travel in ft/sec? A. 122.17 B. 1466.04 C. 100 D. 39.05
MECH 312 RROCABERTE © 2013
v=rω v=14in(1000rev/min)(1ft/12in)(2pi/1rev)(1min/60sec) v=122.17ft/s
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23. When the angular velocity of a 4-ft diameter pulley is 3 rad/s, the total acceleration of a point on its rim is 30 fps2. Determine the angular acceleration of the pulley at this instant?
A. 9 rad/s2
B. 12 rad/s2
C. 15 rad/s2
D. 14 rad/s2
MECH 312 RROCABERTE © 2013
2
222
2
/12 sradα
αra
aaa
ωra
T
nt
N
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24.. A flywheel rotating at 200 rev/min slows down at a constant rate of 2 rad/s2. How many revolutions does it make in the process?
A. 10.47 B. 17.5
C. 109.7 D. 62.83
There’s no other way but up!
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Work – energy required to move an object by a distance
Potential Energy – energy possessed by an object by virtue of its motion (energy at rest), PE = mgh
Kinetic Energy – energy possessed by an object by virtue of its motion (energy in motion), KE = ½ mv2
Power – rate at which energy/work is transferred or consumed
Recall:
Fvdt
dsF
time
WorkP
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Work – product of the component of force in a direction of displacement
Recall:
θ θ
P
θPsWork cos
+W if direction of W is in the direction of displacement -W if direction of W is opposite the direction of displacement
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• Relates the force, displacement and velocity in a given system
• Removes the analysis of internal forces like tension
Work-Energy Method
Principle: The work done in a translating body is equal to the
sum of the change in kinetic energy of the body.
ifnet KEKEKEW
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Derivation of Formula
ds
dvva
dvvdsa
ag
WF
KEsF
vvmsF o
22
2
1
22
00
2o
v
v
s
vvg
WsF
dvvg
WdsF
ds
dvv
g
WF
resultant work
change in kinetic energy
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Practice Problem: Suppose a 30.0-kg package on the roller belt conveyor system is moving at 0.500 m/s. What is its kinetic energy?
Ans. 3.5 J
KE = 1/2mv 2 KE=0.5(30.0 kg)(0.500 m/s) 2
KE=3.75 kg⋅m2/s 2 =3.75 J.
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Practice Problem: Suppose that you push on the 30.0-kg package with a constant force of 120 N through a distance of 0.800 m, and that the opposing friction force averages 5.00 N. (a) Calculate the net work done on the package. (b) Solve the same problem as in part (a), this time by finding the work done by the applied force. Find the work done by the friction.
Ans. (a) 92.0 J; (b) 96.0 J; (c) -4.0 J
(b) The applied force does work: Wapp = Fappdcos(0º)=Fappd = (120 N)(0.800 m) = 96.0 J
(c) So the amounts of work done by gravity, by the normal force, by the applied force, and by friction are, respectively Wgr = 0 Wn = 0 Wapp = 96.0 J Wfr= −4.00 J The total work done: W total= Wgr + Wn + Wapp + Wfr = 92.0
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Energy Stored in a Spring
F = axial force
x = elongation
k = spring constant
2
2
1
2
1kxEFxE springspring
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25. A large coil spring with a spring constant k = 120 N/m is elongated, within its elastic range by 1 m. Compute the stored energy of the spring in N-m.
A. 60
B. 40
C. 20
D. 120
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26. The combined mass of car and passengers travelling at 72 km/hr is 1500kg. Find the kinetic energy of this combined mass.
A. 300kJ
B. 330kJ
C. 305kJ
D. 310kJ
K=mv2 = (1500kg)(72km/hr)2 (1000m/km)2 2k (2)(1kg-m/N-sec2 )(3000 sec/hr)2 = 300,000 J or 300 kJ
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27. A Foucault pendulum swings to 3.0 in above the ground at the highest points and is practically touching the ground at the lowest point. What is the maximum velocity of the pendulum?
A. 4 ft/s C. 5 ft/s
B. 2 ft/s D. 10 ft/s
lost gainedPE =KE
v = 2gh
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UNIT 7
Rowel Allan Rocaberte, REE, ECE [email protected]
+63926 – 740 – 1530
MECH 321: DYNAMICS OF RIGID BODIES (1st Sem 2013-2014)
Front slide photo taken from http://origami-blog.origami-kids.com/paper-airplanes-glossary-of-terms.htm
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Definition
Momentum
- Measure of the motion of a body
- Quantity of motion that an object has
- Product of mass and velocity
Impulse
- Produced when force is applied over time periods
- Product of force and the time
MECH 312 RROCABERTE © 2013
mvP
FtI
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Impulse-Momentum Theorem
Impulse equals the change in momentum
MECH 312 RROCABERTE © 2013
12 vvmFt
dt
dv
g
WF
dt
dva
ag
WF
o
v
v
t
vvg
WtF
dvg
WdtF
dvg
WdtF
o
0
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Law of Conservation of Momentum
Special Case (when P and KE are conserved) :
u2 – u1 relative velocity of separation after collision
v1 – v2 relative velocity of approach before collision
MECH 312 RROCABERTE © 2013
collisionaftercollisionbefore PP
1221 uuvv D6.
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Coefficient of Restitution
The coefficient of restitution (e), or bounciness
of an object is a fractional value representing the
ratio of relative velocities after and before an impact.
An object with an e of 1 collides elastically, while an object with an e < 1 collides inelastically.
MECH 312 RROCABERTE © 2013
Coefficient of Restitution
Collision Remark
e = 1 Perfectly elastic P and KE is conserved
e < 1 Inelastic collision P is conserved; KE is not conserved
e = 0 Perfectly plastic P and KE is not conserved (e.g. colliding particles stick together)
21
12
vv
uue
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21
12
vv
uue
1
2
tan
tan
e
o
r
h
he
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Sign Convention:
Before Collision
After collision
Law of Conservation of Momentum
Loss in Kinetic Energy
MECH 312 RROCABERTE © 2013
100%
initial
initialfinal
loss
initialfinalloss
KE
KEKEKE
KEKEKE
m1 m2
m1 m2
v1 v2
u2 u1
+v -v
22112211 umumvmvm
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28. If the coefficient of restitution is zero, the impact is __________.
A. partially plastic
B. perfectly plastic
C. perfectly elastic
D. partially elastic
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Guide Question
29. A ball is dropped from a height of 20 m upon a stationary slab. If the coefficient of restitution is 0.40, how high will the ball rebound?
A. 3.2 m
B. 4.6 m
C. 5.2 m
D. 8.0 m
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30. The 10 kg and 20 kg bodies are approaching each other with the velocities shown. If e = 0.60, what will be the velocity of each body directly after impact? (a) u1 = -13.33 m/s, u2 = 10 m/s (b) u1 = 6.0 m/s, u2 = 7.5 m/s (c) u1 = 10.25 m/s, u2 = 10 m/s (d) u1 = -13.33 m/s, u2 = 16.7 m/s
MECH 312 RROCABERTE © 2013
21
12
vv
uue
signstheofnote
umumvmvm 22112211
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31. A 200-lb block in contact with the ground is acted upon by a horizontal force equal to 100 lb. The coefficient of kinetic friction is 0.20. In what time will the velocity of the block be increased from 4 fps to 10 fps?
A. 0.621 s
B. 5.1 sec
C. 1.23 sec
D. 8 ms
MECH 312 RROCABERTE © 2013
sec621.0
12
t
vvmtF
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32. A gun is shot in a 0.50 kN block which is hanging from a rope of 1.8 m long. The weight of the bullet is equal to 5 N with a muzzle velocity of 320 m/s. How high will the block swing after it was hit by the bullet?
A. 0.51 m
B. 0.53 m
C. 0.32 m
D. 0.12 m
m1v1 + m2v2 = m1u1 + m2u2
m1v1 + m2v2 = mTu
500(0) + (5/9.81)(320) = (500+5)/9.81 x u u = 3.168 m/s PE = KE mgh = mu2/2 h = u2/2g h = 3.1682/(2x9.81) = 0.512 m
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UNIT 8
Rowel Allan Rocaberte, REE, ECE [email protected]
+63926 – 740 – 1530
MECH 321: DYNAMICS OF RIGID BODIES (Summer 2013)
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Frequency of SHM
Simple
pendulum
Conical
pendulum
L
h
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Frequency of SHM
Helical
spring
in SHM
m
k = spring constant
m = mass
a= acceleration
x = displacement
General
formula
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33. Calculate the period of a simple pendulum connected to a string of length 2 meters.
A. 2.1 sec C. 1.9 sec
B. 1.7 sec D. 2.8 sec
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34. What is the frequency of a 0.6-kN/m helical spring in simple harmonic motion if a body of 3 kg is attached to its end.
A. 1.75 Hz
B. 2.0 Hz
C. 2.25 Hz
D. 1.9 Hz