Momentum & Impulse Conceptual Questions

Momentum & Impulse Conceptual Questions

Let's review the conceptual questions that were homework this week.

These multiple-choice questions are based on the conceptual questions from the HW.

Multiple Choice

We claim that momentum is conserved, but moving objects usually slow down and come to a stop. Why is this?

• Object naturally lose momentum because of Newton's 1st Law

• Momentum of the object runs out and dissipates into the air

• Momentum is only conserved in objects that aren't moving

• Friction exerts a negative impulse which removes momentum

Momentum is conserved when there are not external forces exerted on the system

• If there was no friction between an object and ground, the object would not just "slow down"

• An object in motion stays in motion!

• This is Newton's 1st Law!

• Because friction is an external force, it exerts an impulse on the object.

Multiple Choice

An astronaut is floating out in space and wants to get back to her spaceship. She isn't near any other object, but she is holding a very heavy hand radio. When she throws the radio...

• The radio will pull her in the same direction she threw it.

• The radio will push back on her with the same size force she exerted on it.

• The radio will soar through space but she will stay where she is because there was no external force on her.

• She will float upward because she is less heavy now.

Multiple Choice

Now consider the astronaut scenario by looking at momentum. If she is floating in space at rest before she throws the radio, and then she throws the radio, giving it a momentum of 50 kgm/s, what will her momentum be?

• 0 kgm/s

• 50 kgm/s

• -50 kgm/s

• less than 50 kgm/s

• more than 50 kgm/s

This is an explosion type scenario!

• Astronaut and radio start out together.

• They have initial momentum of ZERO because they are not moving.

• Law of Conservation says momentum before and after must be the same

• So, if momentum of radio is +50 kgm/s afterwards, astronaut momentum must be -50 kgm/s

• -50 + 50 = ZERO!

• This is related to man on the ice scenario!

Multiple Select

You may choose more than one answer: The purpose of a "Crumple zone" on a car is to ____of the collision.

• Decrease the time

• Increase the time

• Decrease the force

• Decrease the impulse

• Increase the energy

Crumple Zones

• INCREASE the time of collision

• DECREASE the force of collision

• The impulse STAYS THE SAME when talking about the same scenario

• This is because the change in momentum with or without crumple zone is the same

• Collisions that take a longer time are safer because force is less!

Multiple Choice

When Superman is shot with a bullet, it bounces off! When a person wearing a bullet proof vest is shot, the bullet is "absorbed" by the vest and stops moving. In which scenario is the impulse larger?

• Superman bouncing back the bullet

• Bulletproof vest stopping the bullet

• Same impulse in both scenarios

• Answer depends on size of the bullet

Bounce --> larger impulse!

• When bullets stop in the vest, just need enough impulse to bring momentum to zero.

• When bullets bounce off Superman, need enough impulse to first STOP the bullets and then turn them around and speed them up again, sending them the other way!

• Impulse = p_f - p_i

• If bullet p_i = 100 kgm/s...

• No Bounce: Impulse = 0 - 100 = -100

• Superman: Impulse = -100 - 100 = -200

Is it possible for a smaller force to exert a larger impulse than a larger force? Yes!!

• Time matters too!

• Notice in the first example that only a 20 N force exerted an impulse of 2,400 kgm/s because the time was very large (120 seconds)

• But in the second example, a gigantic force of 40,000 N only exerted an impulse of 2,000 kgm/s because the time was so incredibly small (0.05 seconds).

Multiple Choice

In which case is the impulse higher -- a car moving at 60 mph slamming on the brakes to stop, or the same speed car coming to a stop by slowly applying the brakes over 30 seconds?

• Slamming on the brakes

• Slowly applying the brakes

• Impulse is the same

• Can't be determined with info given

Impulse = Change in Momentum!

Because the change in momentum is same for both cars here (same mass car going same speed both then coming to a stop), the impulse is the same for both scenarios.