chapter 2 kinematics rectilinear motionhuangj/documents/lecturechap2oned... · 2019-08-30 ·...
TRANSCRIPT
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Chapter 2Kinematics – Rectilinear Motion
•Displacement, Velocity & Acceleration
•Equations describing motion along straight line
•Objects falling freely under earth‟s gravity
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MECHANICS
DYNAMICS(Force & Response)
STATICS(At rest)
KINEMATICS(describes motion)
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Concepts to study objects in motion
Position
Displacement (similar to distance)
Velocity (similar to speed)
Acceleration
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What is “Position” in Physics?
Frame of reference
A choice of coordinate axes with an origin
Coordinates for representing starting and ending points of a motion
One dimension only (this chapter)
(example) a straight line extending to the right and left of the point at the center (origin) and called the x direction
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Displacement
Defined as the change in position
f - final position; i - initial
Units -meters (SI)
CAN BE POSITIVE OR NEGATIVE ALONG A GIVEN DIRECTION !!!
f ix x x
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From A to B xi = 30 m xf = 52 m x = 22 m The displacement is
positive, indicating the motion was in the positive x direction
From C to F xi = 38 m xf = -53 m x = -91 m The displacement is
negative, indicating the motion was in the negative x direction
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Some examples of displacement
Initial x Final x ∆xSign of ∆x
30 52 22 +
38 -53 -91 -
-50 -30 20 +
42 31 -11 -
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Displacement is a “vector”
(Need both magnitude (size) and direction to completely describe)
For 1 D motion just + or – sign
(Scalar quantities have just magnitude)
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IMPORTANT NOTE
Motion along one dimension may be either along x or y or z axis direction and can be used to represent horizontal side to side (motion along x axis) or front and back (motion along z axis) or vertical up and down motion (motion along y axis)
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Displacement not always ≠ distance
Throw a ball straight up and then catch it at the same point you released it
Distance = 2 times height reached by ball
The displacement is zero!!!!!!
so is zeroi fx x x
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Two dimensions Example
East
Walk 3 blocks east and then 4 blocks north
Total distance = ?
Displacement = ?
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Speed
The average speed of an object is defined as the total distance traveled divided by the total time elapsed
SI units – meters/second m/s
Speed - scalar? Vector?
Speed always positive?
total distanceAverage speed
total time
dv
t
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You drove to Trenton campus 8 miles. You drove for 17 minutes and stopped for 3 mins. at a gas station on your way. What was your :
Displacement?
Distance covered?
Average speed?
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+8 miles relative to origin at WWC
8 miles
0.4 miles/min or 10.7 m/s
Think!
What is the displacement for the round trip if you returned back to college?
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Velocity
Average velocity =
SI units – m/s (other units feet/s, cm/s)
Velocity can be positive or negative
Velocity is a vector! For 1D, just +ve or negative
Direction of velocity same as displacement
f iaverage
f i
x xxv
t t t
displacement
time elapsed
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You drove to Trenton campus 8 miles. You drove for 17 minutes and stopped for 3 mins. at a gas station on your way. What was your average velocity?
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Average velocity =+ 10.7 m/s
What is the average velocity for the round trip?
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Speed vs. Velocity
average velocity = average speed?
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Note: In text, bars over physical quantities represent average values.
e. g. average velocity, average speed, average acceleration
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Quick Quiz 2.1
Total distance =
Displacement =
Average velocity in x direction =
Average Speed =
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200 yards, 0, 0, 200÷25 = 8 yards/sec
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Chapter 2 Lecture 2
understand changing velocity & acceleration;
motion with constant acceleration
special case of constant acceleration: free fall
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Understanding Velocity & Acceleration from Graphs
Position(x) vs. time (t) & velocity(v) vs. time (t) graph
Object moving with a constant velocity (uniform motion) – what is the shape of x –t graph?
Average velocity = slope of line joining initial & final positions
Slope of a line is rise divided by run
change in vertical axisslope
change in horizontal axis
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comparing constant & changing velocity
equal displacements in equal intervals of time; average velocity is constant all the time; uniform motion
average velocity (slope of curve) changes from time to time; change in velocity means acceleration!
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Instantaneous velocity
For non uniform velocity, use instantaneous velocity (called simply v) instead of vavg
The limit of the average velocity as the time interval becomes infinitesimally short, or as the time interval approaches zero
instantaneous velocity indicates what is happening at every point of time when velocity keeps changing
lim
0t
xv
t
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Instantaneous Velocity on a Graph
The slope of the line tangent to the position-vs.-time graph is defined to be the instantaneous velocity at that time
The instantaneous speed is defined as the magnitude of the instantaneous velocity
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Acceleration
This is average acceleration
Units are m/s² (SI), cm/s² (cgs), and ft/s² (US Customary)
Vector? Yes, in 1D + or -
f i
f i
v vva
t t t
change in velocityAcceleration
time elapsed
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What are vi and vf ?
Instantaneous values of velocities (initial and final)
Class Example 1
A car takes 2 seconds to accelerate from an initial velocity of +10 m/s to a final velocity +20 m/s. What is the average acceleration?
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ā =+5 m/s2
acceleration is positive and in the same direction as initial velocity
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Class Example 2
A plane travelling at +30 m/s on the runway stops in 5 seconds. What is the acceleration?
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ā = -6 m/s2
(a is negative and it has a sign opposite to that of initial velocity)
TRUE OR FALSE
“So, to slow down an object you always
need to have negative acceleration. To speed up an object, you always need to have positive acceleration.”
FALSE, FALSE, FALSE, FALSE, FALSE, FALSE
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Example: Motion Diagram of A Car
Please insert active figure 2.12
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Average Acceleration
When the sign of the initial velocity and the acceleration are the same (either positive or negative), then the speed is increasing
When the sign of the initial velocity and the acceleration are in the opposite directions, the speed is decreasing
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Negative Acceleration
A negative acceleration does not necessarily mean the object is slowing down
If the acceleration and velocity are both negative, the object is speeding up
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Finding Average Acceleration From v-t graph
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Relationship Between Acceleration and Velocity
Uniform velocity (shown by red arrows maintaining the same size)
Acceleration equals zero
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Relationship Between Velocity and Acceleration
Velocity and acceleration are in the same direction
Acceleration is uniform (blue arrows maintain the same length)
Velocity is increasing (red arrows are getting longer)
Positive velocity and positive acceleration
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Relationship Between Velocity and Acceleration
Acceleration and velocity are in opposite directions
Acceleration is uniform (blue arrows maintain the same length)
Velocity is decreasing (red arrows are getting shorter)
Velocity is positive and acceleration is negative
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Quick Quiz 2.2
True or False?
a) A car must always have an acceleration in the same direction as velocity
b) It is possible for a slowing car to have +veacceleration
c) An object with constant nonzero acceleration can stop but then can not remain at rest
Answers:
a. Think about class example 2
b. Think about slowing car with initial velocity of -10 m/s and final velocity -5 m/s
c. Think about +ve velocity, -ve acceleration of chalk thrown vertically upward
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1D motion with const. acceleration
Equations with constant acceleration, a
Objects thrown up or down on earth move with a special constant acceleration called “g” the acceleration due to gravity = 9.8 m/s2
2
2 2
1
2
1
2
2
o
o
o
o
v v at
x vt v v t
x v t at
v v a x
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Notes on the equations
t2
vvtvx fo
average
If initial t=0 & final t=t, then ∆t = t
Gives displacement as a function of velocity and time
Use when you don‟t know and aren‟t asked for the acceleration
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Notes on the equations
Shows velocity as a function of acceleration and time
Use when you don‟t know and aren‟t asked to find the displacement
ov v at
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Notes on the equations
Gives displacement as a function of time, velocity and acceleration
Use when you don‟t know and aren‟t asked to find the final velocity
2
o at2
1tvx
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Notes on the equations
Gives velocity as a function of acceleration and displacement
Use when you don‟t know and aren‟t asked for the time
2 2 2ov v a x
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Textbook Example 2.4, page 37, Daytona 500
Starts from rest: at t=0, v is zero (v0 = 0)
Constant acceleration: uniform a = 5m/s2
Velocity of car after a distance of 100 ft =?
Use equation:
For distance convert feet to m! (1 m = 3.281 feet)
Time elapsed=?
Use equation:
2 2
02a xv v
0v atv
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Textbook example 2.4 Ans: 17.5 m/s & 3.50 s
End of Chapter MC1:
(!!! sig fig & units!!!)
Choose upward vertical direction as +ve y axis
(coming up: free fall, use „y‟ axis)
Initial velocity +15
Final velocity -8
(Assume a = -9.8 ….coming up)
Find t
Use 0
v atv
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2.35 s
Problem 9
Instantaneous velocities of tennis player at
a) 0.50 s
b) 2.0 s
c) 3.0 s
d) 4.5 s
Just find slope of graph around each t value
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4, -4, 0, 2 with sig fig & units
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Galileo Galilei
1564 - 1642
Galileo formulated the laws that govern the motion of objects in free fall
Also looked at:
Inclined planes
Relative motion
Thermometers
Pendulum
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Free Fall
All objects moving under the influence of gravity only are said to be in free fall
Free fall does not depend on the object‟s original motion
All objects falling near the earth‟s surface fall with a constant acceleration
The acceleration is called the acceleration due to gravity, and indicated by g
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Acceleration due to Gravity
Symbolized by g
g = 9.80 m/s² pointing towards the earth g 10 m/s2
g is always directed downward Toward the center of the earth
Ignoring air resistance and assuming gdoesn‟t vary with altitude over short vertical distances, free fall is constantly accelerated motion
Who is in free fall?
Space shuttle!!!
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Free Fall – an object dropped
Initial velocity is zero
Let up be positive
Use the kinematic equationsTip: Generally use y
instead of x since vertical
Acceleration is g= -9.80 m/s2
vo= 0
a = g
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Free Fall – an object thrown downward
a = g = -9.80 m/s2
Initial velocity 0
With upward being positive, initial velocity will be negative
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Free Fall -- object thrown upward
Initial velocity is upward, so positive
The instantaneous velocity at the maximum height is zero
a = g = -9.80 m/s2
everywhere in the motion
v = 0
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Thrown upward, cont.
The motion may be symmetrical
Then tup = tdown
Then v = -vo
The motion may not be symmetrical
Break the motion into various parts
Generally up and down
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Problem 45: thrown upward with speed 25
This is free fall motion with uniform a=g=9.8
Upward direction is +ve y axis & use g=-9.8
a) Maximum height reached
At this value of ∆x, final v = 0; initial v = 25
Use
b) Use
2 2
02a yv v
0v atv
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31.9m, 2.55s, same t, same initial v & opposite
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Class Example
Obama throws a rock down with speed of 12 m/s
from the top of a tower. The rock hits the ground after 2.0s. What is the height of the tower?
Choose down as positive & choose ∆y for displacement
Given initial velocity (v0=12)
Given time (t = 2)
Given free fall (moving under constant downward acceleration (a=g=9.8)
Asked to find ∆y - the distance traveled by rock from top to bottom (height of tower)
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Use 2
0
1
2y t av t
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44 m (rounded)
In free fall motion of chalk, there is constant acceleration called „g‟ of magnitude 9.8 meter per second squared throughout its trip from the time it leaves my hands until it hits the ground
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Non-symmetrical Free Fall
Need to divide the motion into segments
Possibilities include
Upward and downward portions
The symmetrical portion back to the release point and then the non-symmetrical portion