Kreutter: Momentum
Lesson 1: Throws and Catches
1.1 Represent and Reason
The table below describes experiments for the motion of a ball. For each experiment, something
happens to the ball and its motion changes. Perform every experiment with the ball and meter stick.
Use the information in the table and your experiments to fill in the blanks in the table below.
a)
b)
c)
Initial motion
r
v
Not moving
–x direction
Final motion
Motion diagram to match
Moving in –y
Moving faster in –y
direction
direction
Moving in –x
Not moving in x
direction
direction
–x direction
d)
Not moving in x
direction
Moving in +x
e)
+x direction
direction
+y direction
f)
Moving in –y
direction
1.2 Represent and Reason
The table below describes the velocity of a ball for different experiments. For each experiment, something
happens to the ball and its velocity changes. Use the information in table to fill in the blanks in the table
below.
Initial
velocity, vi
r
v
Final
velocity,
a)
b)
c)
d)
Adapted from PUM: Momentum
©2010 Rutgers, The State University of New Jersey
Motion diagram to match
vf
Kreutter: Momentum
e)
f)
r
g) Which has a larger v : a tank that speeds up from 0 to 1 m/s or a softball pitched underhand?
Explain.
r
h) Which has a larger v : a tank that accelerates from 12 m/s to 13 m/s in 10 seconds or a bike that
accelerates from 2 m/s to 3 m/s in 0.5 seconds? Explain.
r
i) Which has a larger v : a tennis ball that falls into mud and stops, or a tennis ball that falls onto
pavement and bounces? Explain.
Imagine a series of experiments. Fill in the table that follows.
Adapted from PUM: Momentum
©2010 Rutgers, The State University of New Jersey
Kreutter: Momentum
Experiment
Sketch the initial state. Draw
velocity arrows for both objects.
Sketch the final state. Draw velocity arrows
for both objects.
a. Albert, on a
skateboard, is holding
a medicine ball. He
throws the ball
forward and rolls
backwards. The initial
speed of the ball is
much larger than
Albert’s speed.
Before throw
After throw
vci  0
vbi  0
vcf 
Describe the direction
and magnitude of the
final velocities of both
interacting objects.
r
Compare the v of
both objects.
Before catch
b. Albert is standing
still and catches the
ball thrown at him
(moving left). He rolls
backwards holding the
ball. His speed (and
the speed of the ball
after he catches it) is
much less than the
speed of the ball
before it hit him.
Describe the direction
and magnitude of the
initial and final
velocities of both
interacting objects.
r
Compare the v of
both objects.
Adapted from PUM: Momentum
©2010 Rutgers, The State University of New Jersey
After catch
vbf 
Kreutter: Momentum
Experiment
Sketch the initial state. Draw
velocity arrows for both objects.
Sketch the final state. Draw velocity arrows
for both objects.
c. Albert is moving to
the right and catches
the ball thrown at him
(moving left). He slows
down after he catches
the ball. Albert and the
ball continue to move
to the right slower
than Albert was
moving before he
caught the ball.
Describe the direction
and magnitude of the
initial and final
velocities of both
interacting objects.
r
Compare the v of
both objects.
Consider the observations above. Find a pattern that describes what happens to the velocity of the two
objects involved in a throw or catch.
1.4 Explain
In the previous activity, you came up with a pattern that describes what happens to the velocity of two
objects involved in a throw or catch. Use Newton’s laws to explain why this pattern makes sense. It may be
useful to include force diagrams and/or motion diagrams.
1.5 Test Your Idea
Adapted from PUM: Momentum
©2010 Rutgers, The State University of New Jersey
Kreutter: Momentum
Use the pattern that you devised in 1.3 to predict the results of the following experiments. Then
perform the experiments and compare your prediction with the outcome of the real experiment.
Albert is standing on a skateboard
and then he jumps off of it toward
the back.
Cart A is loaded with a block of metal and has
a velcro pad on the front. It moves slowly to
the left on a low-friction track, and hits an
empty and stationary cart B, which has a
velcro patch on the side facing cart A.
Describe the
prediction in
words and a
sketch.
Explain how
you made the
prediction
using the
pattern in 1.3.
Compare the
prediction to
the outcome
of the
experiment.
Discuss whether the pattern you devised in 1.3 was successful in predicting the results of these new
experiments. What judgment can you make about this pattern?
What is the fundamental physics principle from which this pattern follows?
Homework
1.6 Explain
Adapted from PUM: Momentum
©2010 Rutgers, The State University of New Jersey
Kreutter: Momentum
Suppose you place a rifle on gliders and pull the trigger. A bullet (mass 0.020-kg) shoots at high speed
(300 m/s) out of the barrel and the rifle (mass 2.0-kg) recoils back in the opposite direction at speed 3.0
m/s. Does this match the pattern you detected in the experiments above? Explain.
1.5 Explain
a) What do you see when somebody fires a handgun in the movies? What happens to the handgun and
the hand holding it? Can you explain this in terms of the pattern we have discovered?
b) What happens in the movies when somebody (hopefully a bad guy) is shot? Can you explain this in
terms of the pattern we have discovered?
1.6 Explain
At the National Transportation Safety test facility, they record the collision of two identical cars initially
moving at 80 km/h (45 mph) toward each other. Immediately after the collision, the cars are at rest stuck
to each other. The velocities before the collision were the same magnitude but in opposite directions.
Explain.
1.7 Explain
You stand at rest on a frozen pond on your ice skates while wearing a backpack. How might you start
moving without pushing off on the ice? Explain.
1.8 Explain
Adapted from PUM: Momentum
©2010 Rutgers, The State University of New Jersey
Kreutter: Momentum
Read the two different scenarios below. Describe what will happen and explain in terms of our new idea:
(1) A moving train car is rolled gently toward another train car that is not moving. The two cars touch
and lock together. What happens next and why?
(2) A small pirate ship fires a large cannon from its rear deck. What happens next and why?
Reflect: What did you learn in this lesson? How are the ideas developed in this lesson
related to Newton’s laws? How are they related to you experience outside of the physics
classroom? Give 3 examples.
Adapted from PUM: Momentum
©2010 Rutgers, The State University of New Jersey