Question 1

Sandra (45 kg) and David (80 kg) stand motionless on frictionless ice and push off from each other. If Sandra moves to the right at $$v_{Sx} = 2.2\ \mathrm{m/s}$$, what is David's velocity after the push?

Accepted Answer

$$v_{Dx} = -1.2\ \mathrm{m/s}$$

Question 2

Which principle allows us to solve for the final velocities of Sandra and David after they push off from each other on frictionless ice?

Accepted Answer

Conservation of momentum

Question 3

A cart of mass $$2\ \mathrm{kg}$$ moving at $$2\ \mathrm{m/s} to $$the left bounces off a wall and moves to the right at $$1\ \mathrm{m/s}. $$What is the cart's change in momentum?

Accepted Answer

$$30\ \mathrm{kg\cdot m/s}$$

Question 4

You need to close a door quickly by throwing an object at it. You have a sticky clay ball and a super-bouncy Superball, both of equal mass and speed. Which should you throw for maximum impulse to the door?

Accepted Answer

Throw the Superball

Question 5

Why does a Superball deliver more impulse to a door than a clay ball when thrown with the same speed and mass?

Accepted Answer

Because the Superball bounces, reversing its velocity and doubling the momentum change

Question 6

A $$1\ \mathrm{kg}$$ box slides at $$3\ \mathrm{m/s} on $$frictionless ice and collides with a stationary $$2\ \mathrm{kg}$$ box, sticking together. What is their final speed?

Accepted Answer

$$1\ \mathrm{m/s}$$

Question 7

A $$1\ \mathrm{kg}$$ box slides at $$3\ \mathrm{m/s} on $$frictionless ice and bounces off a stationary $$2\ \mathrm{kg}$$ box, reversing direction at $$-3\ \mathrm{m/s}. $$What is the final momentum of the system?

Accepted Answer

$$3\ \mathrm{kg\cdot m/s}$$

Question 8

Two boxes, $$1\ \mathrm{kg}$$ and $$2\ \mathrm{kg}$$, are at rest on a frictionless surface. An explosion pushes the $$1\ \mathrm{kg}$$ box to the left at $$4\ \mathrm{m/s}. $$What is the velocity of the $$2\ \mathrm{kg}$$ box?

Accepted Answer

$$2\ \mathrm{m/s}$$\ to\ the\ right

Question 9

Which type of collision conserves both momentum and kinetic energy?

Accepted Answer

Elastic collision

Question 10

Two railroad cars, $$2.0 	imes 10^4$\ \mathrm{kg}$$ and $$4.0 	imes 10^4$\ \mathrm{kg}$$, move toward each other. The lighter car moves at $$1.5\ \mathrm{m/s} to $$the right, and after coupling, the combined mass moves at $$-0.25\ \mathrm{m/s} to $$the left. What was the initial speed of the heavier car?

Accepted Answer

$$1.1\ \mathrm{m/s}$$\ to\ the\ left

Question 11

A $$0.15\ \mathrm{kg}$$ baseball is thrown at $$20\ \mathrm{m/s}$$ and hit back at $$40\ \mathrm{m/s} by a $$bat in $$0.060\ \mathrm{s}. $$What is the average force exerted by the bat?

Accepted Answer

$$30,000\ \mathrm{N}$$

Question 12

A $$70\ \mathrm{kg}$$ driver traveling at $$20\ \mathrm{m/s}$$ crashes into a row of barrels, stopping in $$2\ \mathrm{s}. $$What is the average force on the driver?

Accepted Answer

$$700\ \mathrm{N}$$

Question 13

A $$0.030\ \mathrm{kg}$$ bullet is fired at $$15\ \mathrm{m/s}$$ from a $$1.2\ \mathrm{kg}$$ rifle initially at rest. What is the recoil speed of the rifle?

Accepted Answer

$$-0.38\ \mathrm{m/s}$$

Question 14

In a perfectly inelastic collision, what happens to the colliding objects?

Accepted Answer

They stick together and move with a common velocity

Question 15

Which of the following is true about momentum in an isolated system during an explosion?

Accepted Answer

Total momentum is conserved

Question 16

A $$1\ \mathrm{kg}$$ box slides at $$4\ \mathrm{m/s} to $$the right, and a $$2\ \mathrm{kg}$$ box slides at $$2\ \mathrm{m/s} to $$the left. They collide and stick together. What is the velocity of the combined mass after the collision?

Accepted Answer

$$0\ \mathrm{m/s}$$

Question 17

Which of the following best describes the impulse delivered to an object?

Accepted Answer

Impulse is the change in momentum, $$\Delta p = F_{avg} \Delta t$$

Question 18

In a collision, why is the change in momentum greater when an object bounces rather than sticks?

Accepted Answer

Because the velocity reverses direction, resulting in a larger $$\Delta p$$

Question 19

A $$1\ \mathrm{kg}$$ box moving at $$3\ \mathrm{m/s}$$ collides with a stationary $$2\ \mathrm{kg}$$ box and bounces back at $$-3\ \mathrm{m/s}. $$What is the velocity of the $$2\ \mathrm{kg}$$ box after the collision?

Accepted Answer

$$1.5\ \mathrm{m/s}$$

Momentum Conservation and Collisions: Study Notes

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