Ch 6 Momentum

1 Мар 2015 | Author: | No comments yet »


Homework, Chapter 6: Momentum

Ch 6; Ex 1, 2, 5, 20, 24, 39, 40, 47, 49

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(Discussion Questions)

Ex 6.1 To bring a to a stop, its engines are typically cut off 25 km from port. Why is it so difficult to or turn a supertanker?

A supertanker has an mass so, with any velocity at its momentum will be very and it will require a force over a very long to bring it to a stop.

Ex 6.2 In terms of and momentum, why are padded dashboards in automobiles?

Impulse = (force)

Impulse = change in momentum

At the of a car crash you have some At the end, your momentum is . This change in momentum is about by a force acting some time. If you run into a steel dashboard, you will be to rest is a very short this means the force on you by the solid steel dashboard be very large. If you run into a dashboard, you will be brought to in a longer time ; this the force exerted on you by a padded will be smaller .

Ex 6.5 In terms of impulse and momentum, why are ropes, which stretch under stress, favored by climbers?

Consider a mountain who falls and is brought to a stop by a rope. Initially the falling has some momentum (mass x After the rope brings the to a stop, the climber’s momentum is As nylon ropes stretch, exert a force over a time so that force be smaller than with a rope which exerts a over a shorter time.

Ex If only an external force can the state of motion of a body, how can the force of the brakes bring a car to

A car moves forward by exerting a force on the road which exerts a (forward) force on the The brakes of a car cause the tires to a force on the road and the road exerts a force on the tires or the This is another example of pairs of forces described by Third Law of Motion.

Ex 6.26 In 5, rocket propulsion was explained in of Newton’s third law. is, the force that propels a is from the exhaust gases against the rocket, the reaction to the the rocket exerts on the exhaust Explain rocket propulsion in of momentum conservation.

Momentum conservation and Newton’s law (action and reaction) are very related. In fact, they are two of describing or explaining the same

Rocket exhaust gases some momentum in one direction and the must carry momentum in the direction to keep the total conserved. Remember, momentum is a . so the direction is very important.

Ex When you are traveling in your car at speed, the momentum of a bug is suddenly as it splatters on your windshield. to the change in momentum of the bug, by how does the momentum of your car

The change in momentum of your car is the as the change in the momentum of the bug — but in the direction. Of course, the bug’s changes enormously because of its mass while your change is very difficult to for it is so small because of the large of your car.

Ex 6.40 If a truck and a Ford Focus a head-on collision, which will experience the greater of impact? The greater impulse? The change in its momentum? The greater

The force of impact is the same .

The is the same .

The change in momentum is the .

Remember, tho’, momentum = (velocity). The Volkswagen has a small so its change in velocity will be this means its acceleration be large. The Mac truck has a very mass so its change in velocity be small; this means its will be small.

Ex 6.47 A is a nuclear particle of unique made up of one proton and one neutron. it is accelerated up to a certain very speed in a cyclotron and directed an observation chamber, where it and sticks to a target particle is initially at rest and then is to move at exactly half the of the incident deutron. Why do observers that the target particle is a deuteron?

momentum before = after

momentum before = before) (velocity before) = after) (velocity after) = after

(mass before) before) = (mass after) after)

( 1 / 2 ) (velocity before) = after

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mass before = ( 1 / 2 ) after)

2 (mass before) = after

That is, the mass the collision — after the deuteron sticks to the target is exactly twice the mass of the Only another deuteron has the of exactly the mass of a deuteron.

Ex 6.49 A billiard ball stop short when it head-on with a ball at The ball cannot stop however, if the collision is not exactly — that is, if the second moves at an angle to the path of the Why? ( Hint: Consider before and after the collision the initial direction of the first and also in a direction perpendicular to initial direction.)

Momentum is a Momentum is conserved along any and the direction perpendicular to that Consider the direction perpendicular to the velocity of the incomming billiard Initially, there is no momentum that direction. But, the collision, if the second billiard moves at some angle, it have momentum along direction. If the incoming billiard simply stopped then would not be conserved. We would, in case, go from having no in this perpendicular direction to momentum in that direction. can not happen. So that means a collision can not happen. * * * Extra * * *

Ex 6.(**extra**) Why is it difficult for a fire to hold a hose that large amounts of water at a speed?

Again, think of Newton’s Law. As water moves the hose at high speed, it great momentum with it and has forces acting upon it. means it will respond by great forces on the hose.

Ex 6. Would you care to fire a gun has a bullet ten times as massive as the

The recoil of a gun is an example of the conservation of Initially, the gun and bullet are at rest and total momentum is zero. the bullet will have a mass and a large velocity the gun, with a much mass, will have a velocity. In this question, the bullet is more massive the gun. That means the of the gun will be greater than the of the bullet. In fact, the velocity of gun will be ten times the velocity of the

Ex 6. (**extra**) The momentum of the rifle and shown in Figure 6.9, on 93, is conserved. Why do we not say the velocities are conserved? would happen if the mass of the were equal to the mass of the

From playing with Third Law, the meaning of and the meaning of momentum, we found the total momentum is conserved the momentum before an interaction, the explosion in firing a bullet a gun, is the same as the momentum the interaction. It is the momentum that is not the velocity. In this particular if the masses are equal, then equal momentum carried to the and to the right does mean the speed to the left will be to the speed to the right. But this is true for this special of equal masses.

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Typical or possible multiple-guess over this material:

1. of the following has the largest momentum to Earth?

A) a tightrope walker Niagara Falls.

B) a pickup speeding along a highway.

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