Lecture 13: Collisions and Impulse
Physics 2210
Fall Semester 2014
Announcement
Exam #2 Wednesday October 29th, 3 PM
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Coverage: Through Unit 13 (today)
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Review on Monday October 27th
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Practice Exam is posted to web page
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Regrade forms from last exam due Friday 24th!
Today's Concepts:
a) More on elastic collisions
b) Average and integrated
force during collisions
Mechanics Lecture 13, Slide 3
Unit 13: Prelecture Feedback
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This section of smartphysics is
conceptual... in class we should
calculate some things.
The 3rd checkpoint problem
Elastic collisions and relative speed
Homework- and exam-type examples
Mechanics Lecture 13, Slide 4
More on Elastic Collisions
In CM frame, the speed of an object before an elastic
collision is the same as the speed of the object after.
m1
v*1, f
m1
v*1,i
m2
v*2,i
m2
v*2, f
General result: rate of approach before an elastic collision
is the same as the rate of separation afterward, in any
reference frame!
Mechanics Lecture 13, Slide 5
Flashcard Question
Consider the two elastic collisions shown below. In
1, a golf ball moving with speed V hits a stationary
bowling ball head on. In 2, a bowling ball moving
with the same speed V hits a stationary golf ball.
In which case does the golf ball have the greater
speed after the collision?
A) 1
B) 2
C) same
V
1
V
2
Mechanics Lecture 13, Slide 6
Flashcard Question
Consider the two elastic collisions shown below. In
1, a golf ball moving with speed V hits a stationary
bowling ball head on. In 2, a bowling ball moving
with the same speed V hits a stationary golf ball.
In which case does the golf ball have the greater
speed after the collision?
A) 1
B) 2
C) same
V
1
V
2
Mechanics Lecture 13, Slide 7
Flashcard Question
A small ball is placed above a much bigger
ball, and both are dropped together from a
height H above the floor. Assume all collisions
are elastic.
To what height do the balls bounce?
H
Before
A
B
After
C
Mechanics Lecture 13, Slide 8
Flashcard Question
A small ball is placed above a much bigger
ball, and both are dropped together from a
height H above the floor. Assume all collisions
are elastic.
To what height do the balls bounce?
H
Before
A
B
After
C
Mechanics Lecture 13, Slide 9
CheckPoint
A block slides to the right with speed V on a frictionless floor and
collides with a bigger block which is initially at rest. After the
collision the speed of both blocks is V/3 in opposite directions. Is
the collision elastic?
A) Yes
B) No
C) Need more information
m
V
M
Before Collision
V/3
V/3
m
M
After Collision
Mechanics Lecture 13, Slide 10
CheckPoint Response
Is the collision elastic?
B) No
C) Need more information
A) Yes
m
V
M
Before Collision
V/3
V/3
m
M
After Collision
A) Yes, the blocks do not stick together.
B) No because the relative speed before the collision
is V and after it's 2V/3 and since those two do not
equal each other the collision is not elastic.
C) The masses of the blocks needs to be known.
Mechanics Lecture 13, Slide 11
Flashcard Question
 
∆P = Favg ∆t
Two identical blocks, B having twice the mass of A, are
initially at rest on frictionless air tracks. You now apply
the same constant force to both blocks for exactly one
second.
F
A
air track
B
F
air track
The change in momentum of block B is:
A) Twice the change in momentum of block A
B) The same as the change in momentum of block A
C) Half the change in momentum of block A
Mechanics Lecture 13, Slide 12
Flashcard Question
 
∆P = Favg ∆t
Two identical blocks, B having twice the mass of A, are
initially at rest on frictionless air tracks. You now apply
the same constant force to both blocks for exactly one
second.
F
A
air track
B
F
air track
The change in momentum of block B is:
A) Twice the change in momentum of block A
B) The same as the change in momentum of block A
C) Half the change in momentum of block A
Mechanics Lecture 13, Slide 13
Flashcard Question
 
∆P = Favg ∆t
Two boxes, one having twice the mass of the other,
are initially at rest on a horizontal frictionless
surface. A force F acts on the lighter box and a
force 2F acts on the heavier box. Both forces act for
exactly one second.
Which box ends up with the biggest momentum?
A) Bigger box
F
B) Smaller box
M
2F
C) same
2M
Mechanics Lecture 13, Slide 14
Flashcard Question
 
∆P = Favg ∆t
Two boxes, one having twice the mass of the other,
are initially at rest on a horizontal frictionless
surface. A force F acts on the lighter box and a
force 2F acts on the heavier box. Both forces act for
exactly one second.
Which box ends up with the biggest momentum?
A) Bigger box
F
B) Smaller box
M
2F
C) same
2M
Mechanics Lecture 13, Slide 15
Example: A ball of mass 0.100 kg is
dropped from a height of 10.0 meters
onto a trampoline. It rebounds to a
height of 7.0 meters, after being in
contact with the trampoline for 0.020
seconds. What is the average force
exerted by the trampoline on the ball?
Similar Example
H
h
vi
The ball is now thrown
with an initial horizontal
velocity = 1.50 m/s to
the right. Does this
change the impulse
that the trampoline
imparts to the ball?
vf
Mechanics Lecture 13, Slide 17
CheckPoint
A constant force acts for a time ∆t on a block that is
initially at rest on a frictionless surface, resulting in a
final velocity V. Suppose the experiment is repeated
on a block with twice the mass using a force that’s
half as big. For how long would the force have to act
to result in the same final velocity?
F
A) Four times as long.
B) Twice as long.
C) The same length.
D) Half as long.
E) A quarter as long.
Mechanics Lecture 13, Slide 18
CheckPoint
A constant force acts for a time ∆t on a block that is
initially at rest on a frictionless surface, resulting in a
final velocity V. Suppose the experiment is repeated
on a block with twice the mass using a force that’s
half as big. For how long would the force have to act
to result in the same final velocity?
F
A) Four times as long.
B) Twice as long.
C) The same length.
D) Half as long.
E) A quarter as long.
Mechanics Lecture 13, Slide 19
Checkpoint #3
Checkpoint #3
HW Problem
Mechanics Lecture 13, Slide 22
Another way to look at it:
Mechanics Lecture 13, Slide 23
HW Problem
Mechanics Lecture 13, Slide 25