Download Momentum Study Guide

Momentum Review Questions
Physics
Study Guide
Momentum
Know the definition of momentum (mass x velocity)
Be able to calculate momentum, mass or velocity given the other two
Be able to explain which of two objects will have more momentum and why
Impulse and Change in Momentum
Know what an impulse is (force exerted x time the force acts)
Know that the impulse on an object is equal to the change in the object's momentum
Be able to find the change in momentum given initial momentum and final momentum
Be able to calculate change in momentum/impulse, force or time given the other two
Be able to explain how to increase the impulse on an object (and therefore change its momentum by a greater amount)
Be able to give and explain situations where you would want a large impulse and how you would get it
Know that for a given impulse (a fixed change in momentum) there are many combinations of force and time that work
Know that for a given impulse, the force and the time that the force acts are inversely related
Be able to explain the different effects of a quick change in momentum versus a more gradual change in momentum for
several different situations
Conservation of Momentum
Know that when two objects collide or interact, the impulse on one object is equal and opposite to the impulse on the other
Know that any momentum lost by one object in a collision or interaction is gained by the other object
Be able to state the law of conservation of momentum: If there are no outside forces acting on a system, the momentum of
the system remains the same.
Be able to explain how the law of conservation of momentum applies to simple collisions and explosions
Be able to find the momentum of an object involved in a collision or explosion based on the law of conservation of
momentum.
Know the difference between elastic and inelastic collisions.
Review Questions
1. Which has more momentum, a cheetah running at 60 miles/hour or a Greyhound bus driving at 60
miles/hour? Why?
2. Which has more momentum, a mosquito flying at 1 m/s or an elephant standing still? Why?
3. Why is it a good idea to follow through on your golf swing? How does that help you hit the golf ball
further?
4. To catch a falling person and absorb their momentum, firefighters often used to use a piece of canvas
stretched on a metal frame. A person can fall from a height of several stories and be unhurt when they
are caught like this. How is this possible? What would happen if the person landed on the sidewalk
instead?
5. A 850,000 kg space shuttle, sitting in the launch cradle, fires its rockets to lift off. The rockets push
straight upward on the ship with 24,000,000 N of force for 8 min.
a) What is the net force on the spaceship? (Don’t forget gravity)
b) How big is the net impulse applied to the spaceship?
c) How fast is the spaceship moving after firing its rockets for 8 min?
d) How high off the ground will the spaceship be at the end of 8 min if it fires its rockets straight up the
whole time?
6. A 10,000 kg bullet train is accelerated forward from rest by a constant force for 70 seconds and ends
up going 90 m/s.
a) How much momentum does the train gain?
b) How big an impulse is applied to the train?
c) How large is the force that is accelerating the train? Assume air resistance and friction are negligible.
7. As Catrina (60 kg) is flying across the continent, a small maintenance oversight leads to a window
popping out and Catrina is shoved out of the window by the pressure difference between the interior of
the cabin and the exterior atmosphere. As she falls, she quickly reaches her terminal velocity of 50 m/s
and then “coasts” down to the ground. As she nears the ground, she sees that she is falling into a
farmer’s field with a haystack, for which she tries to aim.
a) If Catrina hits the haystack, she comes to a stop in 0.08 s. How much force does the hay exert on her
to make her stop?
b) If Catrina lands in the field next to the haystack, she comes to a stop in 0.004 s. How much force
does the ground exert on her to make her stop?
c) Which way does she lose a greater amount of momentum, hitting the hay or hitting the ground?
8. Two cars with the same mass and the same velocity are playing chicken and choose not to swerve.
They crash into each other and get tangled together. What are the cars doing after the collision?
Explain based on the law of conservation of momentum.
9. A large (100 kg) hockey player going 4.8 m/s skates into a smaller player (75 kg) who is at rest.
After the elastic collision, the smaller player is going 4 m/s. How fast is the large player moving?
10. Nick (85 kg) is standing on his skateboard (2 kg) when Alex (60 kg) runs up from behind the
skateboard at 4 m/s and jumps on. Assuming that they don’t fall off and get scraped up badly, how fast
will they be moving afterwards?
11. A small train car has a cannon mounted on it and a total mass of 6000 kg. The car is moving
forward along the tracks at 5 m/s, when the cannon accidentally discharges (good thing there were no
other cars in front of it!). A 20 kg cannonball is shot forward at 250 m/s (compared to the ground).
How fast is the train car moving after the cannonball has been fired?
12. Jeff (70 kg) takes up roller dodge ball - playing dodge ball on roller skates. He is standing perfectly
still facing north when a 1 kg ball hits him going 30 m/s to the south. After being hit, he is rolling
backwards at 0.6 m/s. What is the ball’s velocity after hitting Jeff?