# Physics Chapter 6

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Test 2

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2000 N

1 m/s

6 m/s

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### When you are in the way of a moving object and an impact force is your fate, are you better off decreasing its momentum over a short time or over a long time?

Decreasing its momentum over a long time is better because by doing so you can reduce the impact force on you.

### In karate, why is a force that is applied for a short time more advantageous?

A force that is applied for a short time is able to deliver a high impact force for the same change in momentum.

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

The momentum is said to be conserved when the momentum of a system remains unchanged in a collision.

### Railroad Car A rolls 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 of the cars are the same and car A stops after the collision, the speed of car B has to be similar to the speed with which car A was travelling before it collided with B.

### In the proceeding question, suppose a ball of putty moving horizontally with 1 kg x m/s of momentum collides and sticks to an identical ball of putty moving vertically with 1 kg x m/s of momentum. Why is their combined momentum not simply the arithmetic sum, 2 kg x m/s, what is the total momentum of the balls of putty before and after the collision?

--> P before collision = 1 kg.m/s + 0 = 1 kg.m/s
^ P before collision = 0 + 1 kg.m/s = 1 kg.m/s
Therefore
--->P after collision = 1 kg.m/s
^ P after collision = 1 kg.m/s

### What is the impulse on an 8-kg ball rolling at 2 m/s when it bumps into a pillow and stops?

Impulse = Impact Force x Collision time
= Change in momentum
= m (vf -vi)
= 8 kg ( 0m/s - 2 m/s )
= -16 kg m/s

### When jumping from a significant height, why is it advantageous to land with your knees bent?

Bent knees will allow more time for momentum to decrease, therefore reducing the force of landing.

### A person can survive a feet-first impact at a speed of about 12 m/s on concrete; 15 m/s on soil; and 34 m/s on water. Why the different values for different surfaces?

The time during which the stopping force acts is different for the different situations. Stopping time is least on concrete and most on water, hence the different impact speeds. So there are three concepts; speed at impact, time of impact, and force of impact—which are all related by the impulse-momentum relationship.

### Would you care to fire a gun that has a bullet 10 times as massive as the gun?

No. The gun would recoil with a speed ten times the muzzle velocity. Firing such a gun in the conventional way would not be a good idea!

### Why are the impulses that colliding objects exert on each other equal and opposite?

Impulse is force  time. The forces are equal and opposite, by Newton's third law, and the times are the same, so the impulses are equal and opposite.

### Why does a baseball catcher/m mitt have more padding than a conventional glove?

Impact with a boxing glove extends the time during which mo¬mentum of the fist is reduced, and lessens the force. A punch with a bare fist involves less time and therefore more force.

### If you throw a ball horizontally while standing on roller skates, you will roll backward with a momentum that matches that of the ball. Will you roll backward if you go through the motions of throwing the ball, but instead hold on to it?

If no momentum is imparted to the ball, no oppositely directed momentum will be imparted to the thrower. Going through the motions of throwing has no net effect. If at the beginning of the throw you begin recoiling backward, at the end of the throw when you stop the motion of your arm and hold onto the ball, you stop moving too. Your position may change a little, but you end up at rest. No momentum given to the ball means no recoil momentum gained by you.

### Your friend says that the law of momentum conservation is violated when a ball rolls down a hill and gains momentum. What do you say?

An impulse is responsible for the change in momentum, resulting from a component of gravitational force parallel to the inclined plane.

### If a mack truck and a Mini copper have a head on collision, which vehicle will the experience the greater force impact? The greater impulse? The greater change in momentum? The greater deceleration?

The magnitude of force, impulse, and change in momentum will be the same for each. The Ford Escort undergoes the greater acceleration because its mass is less.

### Judy, mass of 40 kg, standing on slippery ice, catches her leaping dog, mass of 15 kg, moving horizontally at 3.0 m/s. Show that the speed of Judy and her dog after the catch is 0.8 m/s.

From the conservation of momentum,
Momentumdog = momentumJudy + dog
(15 kg)(3.0 m/s) = (40.0 kg + 15 kg)v
45 kgm/s = (55 kg)v
v= 0.8 m/s

### A 5kg fish swimming 1 m/s swallows an absentminded 1 kg fish swimming toward it at speed that brings both fish to a halt immediately after lunch. Show that the speed of the approaching smaller fish before lunch must have been 5 m/s.

Momentumbefore = momentumafter
(5 kg)(1 m/s) + (1 kg)v = 0
-5 kgm/s = (1 kg)v
v= -5m/s
So if the little fish approaches the big fish at 5 m/s, the momentum after lunch will be zero.

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