Physical Science - Chapter 5

For the same force, which cannon imparts the greater speed to a cannonball - a long cannon or a short one? Explain.

A cannonball will have more momentum coming from a long cannon due to the force acting for more time.

How can a supertanker have a huge momentum when it moves relatively slowly?

It has a huge momentum because it has a large mass.

What does it mean to say that momentum (or any quantity) is conserved?

It means that in any interaction where external impulses don't occur, the momentum of the system remains the same before and after the interaction.

a collision in which colliding objects rebound without lasting deformation or the generation of heat

elastic collision

the product of the force acting on an object and the time during which it acts; in interaction, they are equal & opposite

impulse

the impulse is equal to the change in the momentum of the object that the impulse acts on; Ft= ∆mv

impulse-momentum relationship

a collision in which the colliding objects become distorted, generate heat, & possibly join together

inelastic collision

the product of the mass of an object & its velocity

law of conservation of momentum

when no external net force acts on an object or a system of objects, no change of this takes place

momentum

Distinguish between an elastic collision and an inelastic collision. For which type of collision is momentum conserved?

An elastic collision is one where rebound is without lasting deformation or the generation of heat. An inelastic collision is one where deformation does occur, or heat is generated. Momentum is conserved for both types.

Why is it a good idea to extend you hand backward when catching a fast-moving baseball with your bare hand?

An extended hand has more time to move backward when the ball is caught.

To impart the greatest momentum to an object, what should you do in addition to exerting the largest force possible?

Extend the time during which the force acts for more impulse.

Why is it incorrect to say that impulse equals momentum?

Impulse equals change in momentum, not momentum itself.

When a cannonball is fired, is momentum conserved for the cannon-cannonball system as a whole?

Momentum is conserved for the cannon-cannonball system, where the forces are internal to the system. Both forces act within the system. Force is from outside the system.

When a cannonball is fired, its momentum does change. Is momentum conserved for the cannonball?

No because an impulse acts on it.

When a cannonball is fired, the cannon recoils. Is momentum conserved for the cannon?

No because an opposite impulse acts on it.

Railroad car A rolls at a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A?

Since the masses are the same, the speed of car B is equal to the initial speed of car A.

Which is the greater change in momentum, a stop of something dead in its tracks, or a stop and then a reversal of direction?

Stopping and reversing is a greater change in momentum.

Which requires the greater impulse, stopping something dead in its tracks, or stopping it and then reversing its direction?

Stopping and then reversing its direction is a greater change in momentum, which requires a greater impulse.

When can the momentum of two moving objects be cancelled?

When their momentum is equal and opposite can they be cancelled.

In boxing, why is it advantageous to roll with the punch?

The force of the punch will be less if momentum changes over a long time.

If you're in a car with faulty brakes and you have to hit something to stop, the momentum will change to zero whether you hit a brick wall or a haystack. So why is hitting a haystack a safer bet?

The impulse will be the same for both but hitting a haystack means a small force and long time.

If the equally massive cars of the previous question stick together after colliding inelastically, how does their speed after the collision compare with the initial speed of car A?

The momentum of car A is transferred, this time to both cars. Since twice the mass is moving after collision, the speed is half.

Which has a greater momentum, a heavy truck at rest or a moving automobile?

The moving automobile has a greater momentum because it is moving.

In karate why is a short time of the applied force advantageous?

The shorter time has a greater force when there is less momentum.

Why is it less damaging if you fall on a mat than if you fall on a solid floor?

The time to come to a stop will be longer on the mat than on the floor.