pe2113-chapter 12 - dynamics - draft1
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Chapter 12 – Kinematics of a Particle
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Classification of Mechanics
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Kinetics
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12.1 Dynamics
• Dynamics deals with the accelerated motion
of a body.
• Dynamics is divided into 2 parts: Kinematics
and Kinetics – Kinematics deals with the geometric aspects
of the motion of a body
– Kinetics deals with analysis of the forces
casing the motion.
• Dynamics of a particle and then dynamics of
a rigid body.
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Newton’s Contribution
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Other Great Masters
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12.2 Rectilinear Kinematics – Continuous Motion
• Rectilinear motion means motion on a straight line
path.
• Recall a particle: has mass but negligible size and
shape.
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RECTILINEAR KINEMATICS: CONTINIOUS MOTION
(Section 12.2)
A particle travels along a straight-line path
defined by the coordinate axis s.
The total distance traveled by the particle, sT, is a positive
scalar that represents the total length of the path over
which the particle travels.
The position of the particle at any instant,
relative to the origin, O, is defined by the
position vector r, or the scalar s. Scalar s
can be positive or negative. Typical units
for r and s are meters (m) or feet (ft).
The displacement of the particle is
defined as its change in position.
Vector form: r = r’ - r Scalar form: s = s’ - s
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VELOCITY
Velocity is a measure of the rate of change in the position of a particle.
It is a vector quantity (it has both magnitude and direction). The
magnitude of the velocity is called speed, with units of m/s or ft/s.
The average velocity of a particle during a
time interval t is
vavg = r / t
The instantaneous velocity is the time-derivative of position.
v = dr / dt
Speed is the magnitude of velocity: v = ds / dt
Average speed is the total distance traveled divided by elapsed time:
(vsp)avg = sT / t
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ACCELERATION
Acceleration is the rate of change in the velocity of a particle. It is a
vector quantity. Typical units are m/s2 or ft/s2.
As the book indicates, the derivative equations for velocity and
acceleration can be manipulated to get a ds = v dv
The instantaneous acceleration is the time
derivative of velocity.
Vector form: a = dv / dt
Scalar form: a = dv / dt = d2s / dt2
Acceleration can be positive (speed
increasing) or negative (speed decreasing).
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SUMMARY OF KINEMATIC RELATIONS:
RECTILINEAR MOTION
• Differentiate position to get velocity and acceleration.
v = ds/dt ; a = dv/dt or a = v dv/ds
• Integrate acceleration for velocity and position.
• Note that so and vo represent the initial position and
velocity of the particle at t = 0.
Velocity:
= t
o
v
v o
dt a dv = s
s
v
v o o
ds a dv v or = t
o
s
s o
dt v ds
Position:
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Derivative of Acceleration
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1. acceleration build-up, with
maximum positive jerk
2. constant maximum
acceleration (zero jerk)
3. acceleration ramp-down,
approaching the desired
maximum velocity, with
maximum negative jerk
4. constant maximum speed
(zero jerk, zero acceleration)
5. deceleration build-up,
approaching the desired
deceleration, with maximum
negative jerk
6. constant maximum
deceleration (zero jerk)
7. deceleration ramp-down,
approaching the desired
position at zero velocity, with
maximum positive jerk
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CONSTANT ACCELERATION
The three kinematic equations can be integrated for the special case
when acceleration is constant (a = ac) to obtain very useful equations.
A common example of constant acceleration is gravity; i.e., a body
freely falling toward earth. In this case, ac = g = 9.81 m/s2 = 32.2 ft/s2
downward. These equations are:
t a v v c o + = yields = t
o
c
v
v
dt a dv o
2 c o o
s
t (1/2) a t v s s + + = yields = t
o s
dt v ds o
) s - (s 2a ) (v v o c
2
o
2 + = yields = s
s
c
v
v o o
ds a dv v
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12.3 Erratic Motion: Graphs
The approach builds on the facts that slope and differentiation
are linked and that integration can be thought of as finding the
area under a curve.
Graphing provides a good way to
handle complex motions that
would be difficult to describe
with formulas.
Graphs also provide a visual
description of motion and
reinforce the calculus concepts of
differentiation and integration as
used in dynamics.
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S-T GRAPH
Plots of position vs. time can be
used to find velocity vs. time
curves. Finding the slope of the
line tangent to the motion curve at
any point is the velocity at that
point (or v = ds/dt).
Therefore, the v-t graph can be
constructed by finding the slope at
various points along the s-t graph.
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V-T GRAPH
Also, the distance moved
(displacement) of the particle is the
area under the v-t graph during time t.
Plots of velocity vs. time can be used to
find acceleration vs. time curves.
Finding the slope of the line tangent to
the velocity curve at any point is the
acceleration at that point (or a = dv/dt).
Therefore, the acceleration vs. time (or
a-t) graph can be constructed by
finding the slope at various points
along the v-t graph.
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A-T GRAPH
Given the acceleration vs. time
or a-t curve, the change in
velocity (v) during a time
period is the area under the a-t
curve.
So we can construct a v-t graph
from an a-t graph if we know the
initial velocity of the particle.
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12.4 GENERAL CURVILINEAR MOTION
A particle moving along a curved path undergoes curvilinear motion.
Since the motion is often three-dimensional, vectors are used to
describe the motion.
The position of the particle at any instant is designated by the vector
r = r(t). Both the magnitude and direction of r may vary with time.
A particle moves along a curve
defined by the path function, s.
If the particle moves a distance s along the
curve during time interval t, the
displacement is determined by vector
subtraction: r = r’ - r
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VELOCITY
Velocity represents the rate of change in the position of a
particle.
The average velocity of the particle
during the time increment t is
vavg = r/t .
The instantaneous velocity is the
time-derivative of position
v = dr/dt .
The velocity vector, v, is always
tangent to the path of motion.
The magnitude of v is called the speed. Since the arc length s
approaches the magnitude of r as t→0, the speed can be
obtained by differentiating the path function (v = ds/dt). Note
that this is not a vector!
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ACCELERATION
Acceleration represents the rate of change in the
velocity of a particle.
If a particle’s velocity changes from v to v’ over a
time increment t, the average acceleration during
that increment is:
aavg = v/t = (v - v’)/t
The instantaneous acceleration is the time-
derivative of velocity:
a = dv/dt = d2r/dt2
A plot of the locus of points defined by the arrowhead
of the velocity vector is called a hodograph. The
acceleration vector is tangent to the hodograph, but
not, in general, tangent to the path function.
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12.5 CURVILINEAR MOTION: RECTANGULAR COMPONENTS
It is often convenient to describe the motion of a particle in
terms of its x, y, z or rectangular components, relative to a fixed
frame of reference.
The magnitude of the position vector is: r = (x2 + y2 + z2)0.5
The direction of r is defined by the unit vector: ur = (1/r)r
The position of the particle can be
defined at any instant by the
position vector
r = x i + y j + z k .
The x, y, z components may all be
functions of time, i.e.,
x = x(t), y = y(t), and z = z(t) .
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RECTANGULAR COMPONENTS: VELOCITY
The magnitude of the velocity
vector is
v = [(vx)2 + (vy)
2 + (vz)2]0.5
The direction of v is tangent
to the path of motion.
The velocity vector is the time derivative of the position vector:
v = dr/dt = d(xi)/dt + d(yj)/dt + d(zk)/dt
Since the unit vectors i, j, k are constant in magnitude and
direction, this equation reduces to v = vx i + vy j + vz k
where vx = = dx/dt, vy = = dy/dt, vz = = dz/dt x y z • • •
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RECTANGULAR COMPONENTS: ACCELERATION
The direction of a is usually
not tangent to the path of the
particle.
The acceleration vector is the time derivative of the
velocity vector (second derivative of the position vector):
a = dv/dt = d2r/dt2 = ax i + ay j + az k
where ax = = = dvx /dt, ay = = = dvy /dt,
az = = = dvz /dt
vx x vy y
vz z
• •• ••
•• •
•
The magnitude of the acceleration vector is
a = [(ax)2 + (ay)
2 + (az)2 ]0.5
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EXAMPLE
Given: The motion of two particles (A and B) is described by
the position vectors
rA = [3t i + 9t(2 – t) j] m and
rB = [3(t2 –2t +2) i + 3(t – 2) j] m.
Find: The point at which the particles collide and their
speeds just before the collision.
Plan: 1) The particles will collide when their position
vectors are equal, or rA = rB .
2) Their speeds can be determined by differentiating
the position vectors.
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EXAMPLE (continued)
1) The point of collision requires that rA = rB,
so xA = xB and yA = yB .
Solution:
Set the x-components equal: 3t = 3(t2 – 2t + 2)
Simplifying: t2 – 3t + 2 = 0
Solving: t = {3 [32 – 4(1)(2)]0.5}/2(1)
=> t = 2 or 1 s
Set the y-components equal: 9t(2 – t) = 3(t – 2)
Simplifying: 3t2 – 5t – 2 = 0
Solving: t = {5 [52 – 4(3)(–2)]0.5}/2(3)
=> t = 2 or – 1/3 s
So, the particles collide when t = 2 s (only common
time). Substituting this value into rA or rB yields
xA = xB = 6 m and yA = yB = 0
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EXAMPLE (continued)
2) Differentiate rA and rB to get the velocity vectors.
Speed is the magnitude of the velocity vector.
vA = (32 + 182) 0.5 = 18.2 m/s
vB = (62 + 32) 0.5 = 6.71 m/s
vB = drB/dt = xB i + yB j = [ (6t – 6) i + 3 j ] m/s
At t = 2 s: vB = [ 6 i + 3 j ] m/s • •
vA = drA/dt = = [ 3 i + (18 – 18t) j ] m/s
At t = 2 s: vA = [ 3i – 18 j ] m/s
j yA i xA . + • •
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12.6 MOTION OF A PROJECTILE
Projectile motion can be treated as two rectilinear motions, one in
the horizontal direction experiencing zero acceleration and the other
in the vertical direction experiencing constant acceleration (i.e.,
from gravity).
For illustration, consider the two balls on the
left. The red ball falls from rest, whereas the
yellow ball is given a horizontal velocity. Each
picture in this sequence is taken after the same
time interval. Notice both balls are subjected to
the same downward acceleration since they
remain at the same elevation at any instant.
Also, note that the horizontal distance between
successive photos of the yellow ball is constant
since the velocity in the horizontal direction is
constant.
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KINEMATIC EQUATIONS: HORIZONTAL MOTION
Since ax = 0, the velocity in the horizontal direction remains
constant (vx = vox) and the position in the x direction can be
determined by:
x = xo + (vox) t Why is ax equal to zero (assuming movement through the air)?
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KINEMATIC EQUATIONS: VERTICAL MOTION
Since the positive y-axis is directed upward, ay = – g.
Application of the constant acceleration equations yields:
vy = voy – g t
y = yo + (voy) t – ½ g t2
vy2 = voy
2 – 2 g (y – yo)
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EXAMPLE II
Given: Projectile is fired with vA=150 m/s
at point A.
Find: The horizontal distance it travels (R) and
the time in the air.
Plan:
Establish a fixed x, y coordinate system (in this solution,
the origin of the coordinate system is placed at A).
Apply the kinematic relations in x- and y-directions.
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EXAMPLE II (continued)
Solving for tAB first, tAB = 19.89 s.
Then, R = 120 tAB = 120 (19.89) = 2387 m
Solution: 1) Place the coordinate system at point A.
Then, write the equation for horizontal motion.
+ xB = xA + vAx tAB
where xB = R, xA = 0, vAx = 150 (4/5) m/s
Range, R will be R = 120 tAB
2) Now write a vertical motion equation. Use the distance equation.
+ yB = yA + vAy tAB – 0.5 g tAB2
where yB = – 150, yA = 0, and vAy = 150(3/5) m/s
We get the following equation: –150 = 90 tAB + 0.5 (– 9.81) tAB2
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APPLICATIONS (continued)
A roller coaster travels down a
hill for which the path can be
approximated by a function
y = f(x).
The roller coaster starts from rest
and increases its speed at a
constant rate.
How can we determine its velocity
and acceleration at the bottom?
Why would we want to know
these values?
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12.7 NORMAL AND TANGENTIAL COMPONENTS
When a particle moves along a curved path, it is sometimes convenient
to describe its motion using coordinates other than Cartesian. When the
path of motion is known, normal (n) and tangential (t) coordinates are
often used.
In the n-t coordinate system, the
origin is located on the particle
(the origin moves with the
particle).
The t-axis is tangent to the path (curve) at the instant considered,
positive in the direction of the particle’s motion.
The n-axis is perpendicular to the t-axis with the positive direction
toward the center of curvature of the curve.
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NORMAL AND TANGENTIAL COMPONENTS (continued)
The position of the particle at any
instant is defined by the distance, s, along the curve from a
fixed reference point.
The positive n and t directions are
defined by the unit vectors un and ut,
respectively.
The center of curvature, O’, always
lies on the concave side of the curve.
The radius of curvature, r, is defined
as the perpendicular distance from
the curve to the center of curvature at
that point.
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VELOCITY IN THE n-t COORDINATE SYSTEM
The velocity vector is always
tangent to the path of motion
(t-direction).
The magnitude is determined by taking the time derivative of
the path function, s(t).
v = v ut where v = s = ds/dt .
Here v defines the magnitude of the velocity (speed) and
ut defines the direction of the velocity vector.
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ACCELERATION IN THE n-t COORDINATE SYSTEM
Acceleration is the time rate of change of velocity:
a = dv/dt = d(vut)/dt = vut + vut
. .
Here v represents the change in
the magnitude of velocity and ut
represents the rate of change in
the direction of ut.
.
.
.
a = v ut + (v2/r) un = at ut + an un.
After mathematical manipulation,
the acceleration vector can be
expressed as:
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ACCELERATION IN THE n-t COORDINATE SYSTEM
(continued)
So, there are two components to the
acceleration vector:
a = at ut + an un
• The normal or centripetal component is always directed
toward the center of curvature of the curve. an = v2/r
• The tangential component is tangent to the curve and in the
direction of increasing or decreasing velocity.
at = v or at ds = v dv .
• The magnitude of the acceleration vector is
a = [(at)2 + (an)
2]0.5
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SPECIAL CASES OF MOTION
There are some special cases of motion to consider.
1) The particle moves along a straight line.
r => an = v2/r = 0 => a = at = v
The tangential component represents the time rate of change in
the magnitude of the velocity.
.
2) The particle moves along a curve at constant speed.
at = v = 0 => a = an = v2/r .
The normal component represents the time rate of change in the
direction of the velocity.
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SPECIAL CASES OF MOTION (continued)
3) The tangential component of acceleration is constant, at = (at)c.
In this case,
s = so + vo t + (1/2) (at)c t2
v = vo + (at)c t
v2 = (vo)2 + 2 (at)c (s – so)
As before, so and vo are the initial position and velocity of the
particle at t = 0. How are these equations related to projectile
motion equations? Why?
4) The particle moves along a path expressed as y = f(x).
The radius of curvature, r, at any point on the path can be
calculated from
r = ________________ ]3/2 (dy/dx)2 1 [ +
2 d2y/dx
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THREE-DIMENSIONAL MOTION
If a particle moves along a space
curve, the n and t axes are defined as
before. At any point, the t-axis is
tangent to the path and the n-axis
points toward the center of curvature.
The plane containing the n and t axes
is called the osculating plane.
A third axis can be defined, called the binomial axis, b. The
binomial unit vector, ub, is directed perpendicular to the osculating
plane, and its sense is defined by the cross product ub = ut × un.
There is no motion, thus no velocity or acceleration, in the
binomial direction.
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EXAMPLE
Given: A boat travels around a
circular path, r = 40 m, at a
speed that increases with
time, v = (0.0625 t2) m/s.
Find: The magnitudes of the boat’s
velocity and acceleration at
the instant t = 10 s.
Plan:
The boat starts from rest (v = 0 when t = 0).
1) Calculate the velocity at t = 10 s using v(t).
2) Calculate the tangential and normal components of
acceleration and then the magnitude of the
acceleration vector.
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EXAMPLE (continued)
Solution:
1) The velocity vector is v = v ut , where the magnitude is
given by v = (0.0625t2) m/s. At t = 10s:
v = 0.0625 t2 = 0.0625 (10)2 = 6.25 m/s
2) The acceleration vector is a = atut + anun = vut + (v2/r)un. .
Tangential component: at = v = d(.0625 t2 )/dt = 0.125 t m/s2
At t = 10s: at = 0.125t = 0.125(10) = 1.25 m/s2
.
Normal component: an = v2/r m/s2
At t = 10s: an = (6.25)2 / (40) = 0.9766 m/s2
The magnitude of the acceleration is
a = [(at)2 + (an)
2]0.5 = [(1.25)2 + (0.9766)2]0.5 = 1.59 m/s2
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APPLICATIONS
The cable and pulley system shown
can be used to modify the speed of
the mine car, A, relative to the speed
of the motor, M.
It is important to establish the
relationships between the various
motions in order to determine the
power requirements for the motor
and the tension in the cable.
For instance, if the speed of the cable (P) is known because we
know the motor characteristics, how can we determine the
speed of the mine car? Will the slope of the track have any
impact on the answer?
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12.10 RELATIVE POSITION
The absolute position of two
particles A and B with respect to
the fixed x, y, z reference frame are
given by rA and rB. The position of
B relative to A is represented by
rB/A = rB – rA
Therefore, if rB = (10 i + 2 j ) m
and rA = (4 i + 5 j ) m,
then rB/A = (6 i – 3 j ) m.
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RELATIVE VELOCITY
To determine the relative velocity of B
with respect to A, the time derivative of
the relative position equation is taken.
vB/A = vB – vA
or
vB = vA + vB/A
In these equations, vB and vA are called absolute velocities
and vB/A is the relative velocity of B with respect to A.
Note that vB/A = - vA/B .
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RELATIVE ACCELERATION
The time derivative of the relative
velocity equation yields a similar
vector relationship between the
absolute and relative accelerations
of particles A and B.
These derivatives yield: aB/A = aB – aA
or
aB = aA + aB/A
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SOLVING PROBLEMS
Since the relative motion equations are vector equations,
problems involving them may be solved in one of two ways.
For instance, the velocity vectors in vB = vA + vB/A could be
written as two dimensional (2-D) Cartesian vectors and the
resulting 2-D scalar component equations solved for up to
two unknowns.
Alternatively, vector problems can be solved “graphically” by
use of trigonometry. This approach usually makes use of the
law of sines or the law of cosines.
Could a CAD system be used to solve these types of problems?
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LAWS OF SINES AND COSINES
a b
c
C
B A
Since vector addition or subtraction forms
a triangle, sine and cosine laws can be
applied to solve for relative or absolute
velocities and accelerations. As a review,
their formulations are provided below.
Law of Cosines: A bc c b a cos 2 2 2 2 - + =
B ac c a b cos 2 2 2 2 - + =
C ab b a c cos 2 2 2 2 - + =
Law of Sines: C
c
B
b
A
a
sin sin sin = =
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EXAMPLE
Given: vA = 650 km/h
vB = 800 km/h
Find: vB/A
Plan:
a) Vector Method: Write vectors vA and vB in Cartesian
form, then determine vB – vA
b) Graphical Method: Draw vectors vA and vB from a
common point. Apply the laws of sines and cosines to
determine vB/A.
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EXAMPLE (continued)
Solution:
vA = (650 i ) km/h
vB = –800 cos 60 i – 800 sin 60 j
= ( –400 i – 692.8 j) km/h
a) Vector Method:
vB/A = vB – vA = (–1050 i – 692.8 j) km/h
v A B /
= (-1050)2 +(-692.8)2 = 1258 km/h
= tan-1( ) = 33.4 1050
692.8
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EXAMPLE (continued)
b) Graphical Method:
Note that the vector that measures the tip of B relative to A is vB/A.
vB
vA
120
vB/A
Law of Cosines:
(vB/A)2 = (800) 2 + (650) 2 − (800) (650) cos 120
vB/A = 1258 km/h
vB/A vA
sin(120 ) sin
= or = 33.4
Law of Sines:
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Rectilinear Kinematics – Continuous Motion
82
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Important Points
• Dynamics is concerned with bodies that have accelerated motion.
• Kinematics is a study of the geometry of the motion.
• Kinetics is a study of the forces that cause the motion.
• Rectilinear kinematics refers to straight-line motion.
• Speed refers to the magnitude of velocity.
• Average speed is the total distance traveled divided by the total
time. This is different from the average velocity, which is the
displacement divided by the time.
• A particle that is slowing down is decelerating.
• A particle can have an acceleration and yet have zero velocity.
• The relationship a ds = v dv is derived from a = dv/dt and v = ds/dt,
by eliminating dt.
83
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Procedures for Analysis • Coordinate System.
– Establish a position coordinate s along the path and specify its fixed origin and
positive direction.
– Since motion is along a straight line, the vector quantities position, velocity, and
acceleration can be represented as algebraic scalars. For analytical work the
sense of s, v, and a is then defined by their algebraic signs.
– The positive sense for each of these scalars can be indicated by an arrow shown
alongside each kinematic equation as it is applied.
• Kinematic Equations.
– If a relation is known between any two of the four variables a, v, sand t, then a
third variable can be obtained by using one of the kinematic equations, a = dv/ dt,
v = ds/ dt or a ds = v dv, since each equation relates all three variables. *
– Whenever integration is performed, it is important that the position and velocity be
known at a given instant in order to evaluate either the constant of integration if
an indefinite integral is used, or the limits of integration if a definite integral is
used.
– Remember that Eqs. 12-4 through 12-6 have only limited use. These equations
apply only when the acceleration is constant and the initial conditions are s = So
and v = Vo when t = 0. 84
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Chap. 13: Kinetics of a Particle
105
F = ma or Fx i + Fy j + Fz k = m(ax i + ay j + az k)
or, as scalar equations, Fx = max, Fy = may, and Fz = maz.
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Example 13.2
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Example 13.2 (continued)
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Example 13.2 (continued)
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13.5 NORMAL & TANGENTIAL COORDINATES
When a particle moves along a curved
path, it may be more convenient to
write the equation of motion in terms of
normal and tangential coordinates.
The normal direction (n) always points toward the path’s center of curvature.
In a circle, the center of curvature is the center of the circle.
The tangential direction (t) is tangent to the path, usually set as positive in
the direction of motion of the particle.
Ftut + Fnun+ Fbub = mat+man
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NORMAL AND TANGENTIAL ACCERLERATIONS
The tangential acceleration, at = dv/dt, represents the time rate
of change in the magnitude of the velocity. Depending on the
direction of Ft, the particle’s speed will either be increasing or
decreasing.
The normal acceleration, an = v2/r, represents the time rate of
change in the direction of the velocity vector. Remember, an
always acts toward the path’s center of curvature. Thus, Fn
will always be directed toward the center of the path.
Recall, if the path of motion is
defined as y = f(x), the radius of
curvature at any point can be
obtained from
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Example 13.9
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Example 13.9 (continued)
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14.1 WORK OF A FORCE
A force does work on a particle when the particle undergoes a displacement
along the line of action of the force.
Work is defined as the product of force and
displacement components acting in the same
direction. So, if the angle between the force and
displacement vector is , the increment of work
dU done by the force is
dU = F ds cos
By using the definition of the dot product and
integrating, the total work can be written as
r2
r1
U1-2 = F • dr
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Principle of Work and Energy
114
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Impulse and Momentum
115