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22SU PHY-1041-VO04 Week #8 HW
Part A: NOTES – Momentum
Use the assigned readings in OpenSTAX Chapter 8, the Momentum Knowledge Organizer posted in the Week 8
Module and Catherine’s Week 8 videos to take notes on the following:
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What is momentum?
o What are the units for momentum?
o Is momentum a vector?
o Give examples of objects with a lot and a little momentum.
o What is the difference between momentum and impulse? Use an example to explain.
When is momentum conserved?
o What does it mean that momentum is conserved?
o Use examples to illustrate when momentum is conserved and not conserved.
What is Newton’s Second Law of Motion stated in terms of momentum?
∆
o Be able to show how Fnet = ma and 𝐹 = are equivalent. Hint: See Section 8.1
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What is the key graph in this unit?
o What information do we get form it?
o Compare this with the key graph in the energy unit. What information did we get from that graph?
What is the difference between an elastic and inelastic collision?
o Give examples.
Part B: PRACTICE with CONCEPTS & CALCULATIONS – Momentum
1. A baseball (m = 0.145 kg) is thrown to the left with a
speed of 37 m/s (85 mph). The batter swings the bat (m = 1
kg) to the right with a speed of 30 m/s (70 mph).
(a) Find the magnitude and direction of the momentum
of the baseball.
(b) Find the magnitude and direction of the momentum
of the bat.
(c) The batter makes contact and hits a homerun. Is this
an elastic or inelastic collision? How do you know?
(d) There is an exchange of momentum between the bat
and the ball when they collide. Which object do you
think has more momentum after the collision? Why?
(e) Is the system of the bat + baseball an example of a system with no external forces? Explain.
(f) Is momentum conserved in the system of the bat + baseball? Explain.
2. Use the concept of impulse to explain why it is painful to jump and land with your knees straight but
comfortable to jump and land with your knees bent.
“Practice is the best of all instructors.” P. Syrus | Latin writer 85-43 BC
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22SU PHY-1041-VO04 Week #8 Homework
3. A graph of force versus time for an object moving to the right is shown below. Is the change in momentum
greater from a time of 0 to 0.2 s or from a time of 0.2 to 0.4 s?
4. Before the 1980’s, cars were made of very rigid materials that did not crumple in an accident. Now, cars are
made so that they have “crumple zones” and “fold up” during accidents. Use the concept of impulse to
explain why this change in car design has saved countless lives.
5. Two carts traveling in opposite
directions are shown just before they
collide. The carts carry different loads
and are initially traveling at different
speeds. When the two carts collide they
lock together.
(a) Is this an elastic or inelastic
collision?
(b) Is momentum conserved in this situation?
(c) Calculate the final velocity of the two carts.
Be gritty. Be curious.
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22SU PHY-1041-VO04 Week #8 Homework
6. A cart is moving to the right when its mass
is suddenly doubled. Assuming ideal
conditions, the final velocity of the cart will
be:
a.
less than the initial velocity
b.
equal to the initial velocity
c.
more than the initial velocity
d.
need more information to determine
Why?
p (kg m/s)
7. The graph to the right shows momentum versus
time for an object moving to the right. It provides a
way to visualize how the momentum is changing over
time for this object.
a) During which interval is there no net force
acting on the object? How do you know?
b) What is the magnitude of the force acting over
the first 10 seconds? How do you know?
c) Is momentum conserved for this object?
Explain.
d) If this object is a soccer ball, describe its motion over these 60 seconds.
8. A 3,000 kg truck has an initial velocity of +10 m/s. It collides with a 1000 kg car initially traveling at + 5
m/s. After the collision, the car is moving with v = +15 m/s. What is the truck’s final velocity? Assume the
surface is ideal and air resistance is negligible.
9. A 3,000 kg truck has an initial velocity of +10 m/s. It collides with a 1000 kg car initially at rest. After the
collision, the car is moving with v = +15 m/s. What is the truck’s final velocity? Assume the surface is ideal and
air resistance is negligible.
10. A 5.0 kg cart is rolling on a table to the right at 10 m/s. It collides with a 2.0 kg cart initially at rest. The
two carts collide completely inelastically. What is the final velocity of the system? Assume the surface is ideal
and air resistance is negligible. Hint: Think about what completely inelastically means about the final
velocities.
11. Scenario: While playing pool, a person strikes the cue ball (the white ball) setting it into motion with a
velocity of 2 m/s. The cue ball collides head on with the green 6 ball. The cue ball stops. With what speed
does the 6-ball move after the collision? Assume both balls have the same mass.
Be gritty. Be curious.
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22SU PHY-1041-VO04 Week #8 Homework
12. Increasing the time of collision mitigates the force of impact. This is illustrated, for example, by how air
bags in a car protect the driver and passengers during a collision. By how much does the force of impact
decrease if the time of collision is quadrupled? You can either solve this completely symbolically, or you can
design your own scenario with mass, velocity and time.
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Part C: PRACTICE with COMBINING CONCEPTS to keep them FRESH!
1. Given the motion graph to the right, calculate:
(a) the change in velocity for this 10 kg object.
(b) the acceleration of this 10 kg object.
(c) the impulse for this 10 kg object.
(d) the net force acting on this object over 5 seconds.
(e) the displacement of this object over 5 seconds.
(f) the change in kinetic energy of this object.
(g) the work done on this object.
2. Examine the position-time graph to the right for objects A and B.
(a) Describe the collision taking place. Assume no external forces are
present.
(b) Sketch a velocity-time graph for this scenario.
(c) Compare the kinetic energies of objects A and B. Assume they are
equal mass.
3. CHALLENGE: A toy manufacturer is advertising a new kind of bouncy ball. They claim it bounces perfectly
100% elastically with any hard surface, such as concrete.
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Describe an experimental procedure you could execute to test this claim.
What would you measure?
How would you use your measurements to test the claim?
Be gritty. Be curious.
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