Physics 110 Chapter 6 & 7

Suppose a ball of putty moving horizontally with 1 kg·m/s of momentum collides with and sticks to an identical ball of putty moving vertically with 1 kg·m/s of momentum. What is the magnitude of their combined momentum?

1.41 kg·m/s

What will the kinetic energy of a pile driver ram be if it starts from rest and undergoes a 10 kJ decrease in potential energy?

10kJ

Suppose we repeat the experiment from the video, but this time we use a rocket three times as massive as the one in the video, and in place of water we use a fluid that is twice as massive (dense) as water. If the new fluid leaves the rocket at the same speed as the water in the video, what will be the ratio of the horizontal speed of our rocket to the horizontal speed of the rocket in the video after all the fluid has left the rocket? (Ignore air resistance.).

2/3

If you push a crate horizontally with 100 N across a 10-m factory floor and the friction between the crate and the floor is a steady 70 N, how much kinetic energy does the crate gain?

300 J

When the useful energy output of a simple machine is 100 J, and the total energy input is 200 J, the efficiency is _______.

50%

If an input of 100 J in a pulley system increases the potential energy of a load by 60 J, what is the efficiency of the system?

60%

A. Find the force acting on the egg when it hits the sheet. B. If the mass of the egg is 1.3 kg , its initial speed is 2.2 m/s , and the time to stop is 0.20 s, find the average force on the egg.

A. F = mv/t B. F = 14 N

Part A: How is Dr. Hewitt able to break a piece of wood in his demonstration? Part B How does Dr. Hewitt break the piece of wood? Part C What would Dr. Hewitt need to have done to exert an even greater force than he did in his karate demonstration?

A. He moves his hand very quickly. B. He causes a change in momentum of his hand over a short period of time. C. Increase the change in momentum, and decrease the time duration

A. A fully dressed person is at rest in the middle of a pond on perfectly frictionless ice and must get to shore. How can this be accomplished? Assuming you've answered them in terms of momentum conservation, answer them also in terms of Newton's third law (or vice versa, if you answered already in terms of Newton's third law). B. If you throw a ball horizontally while standing on roller skates, you 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? Explain. Assuming you've answered them in terms of momentum conservation, answer them also in terms of Newton's third law (or vice versa, if you answered already in terms of Newton's third law).

A. He/She could take off their shoe and throw it in the opposite direction that they want to go in since the conservation of momentum states that they will move in the opposite direction on the slippery ice. B.To break it down, no momentum is imparted to the ball, therefore no oppositely directed momentum will be imparted to the thrower. So 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, it stops moving too. The position may change a little, but you should end up at rest. With no momentum given, no recoil momentum is gained by the you.

A. Which word in the statement of this problem allows you to assume that the table is frictionless? B. Suppose the potential energy of the block at the table is given by mgh/3. This implies that the chosen zero level of potential energy is __________. C. If the zero level is a distance 2h/3 above the floor, what is the potential energy U of the block on the floor? D. Considering that the potential energy of the block at the table is mgh/3 and that on the floor is −2mgh/3, what is the change in potential energy ΔU of the block if it is moved from the table to the floor? E. Which most simplified form of the law of conservation of energy describes the motion of the block when it slides from the top of the table to the bottom of the ramp? F. As the block slides down the ramp, what happens to its kinetic energy K, potential energy U, and total mechanical energy E? G.Using conservation of energy, find the speed vb of the block at the bottom of the ramp. H.Which most simplified form of the law of conservation of energy describes the motion of the block as it slides on the floor from the bottom of the ramp to the moment it stops? I.As the block slides across the floor, what happens to its kinetic energy K, potential energy U, and total mechanical energy E? J.What force is responsible for the decrease in the mechanical energy of the block? K. Find the amount of energy E dissipated by friction by the time the block stops.

A. Smooth B. a distance 2h/3 above the floor C. U = −2mgh/3 D. ΔU = −mgh E. 12mv2i+mghi=12mv2f+mghf F. K increases;U decreases;E stays the same G. vb = √v^2+2gh H. 12mv2i+Wnc=0 I. K decreases;U stays the same;E decreases J. friction. K. E = 0.5mv2+mgh

(Part A) What happens the first time Dr. Hewitt lifts the bowling ball near his teeth and lets go? (Part B) Why does the bowling ball behave the way it does the first time Dr. Hewitt lifts the bowling ball near his teeth and lets go? (Part C) What happens the second time Dr. Hewitt lifts the bowling ball near his teeth and gives it a push? (Part D) Why does the bowling ball behave as it does when Dr. Hewitt lifts it and gives it a push?

A. The ball returns to Dr. Hewitt, stopping almost exactly at the point where it was released. B. All of the initial energy of the ball was converted completely back to potential energy when the ball returned. C.The ball leaves Dr. Hewitt and returns to him, going past the point where it was released. D. The extra energy from the push is converted into kinetic energy, which is then converted into more potential energy at the end of the motion than the ball had when it was released.

Part A Before you click "Play," predict what will happen if the red and green truck with equal mass and equal magnitude of velocity collide. Part B Predict what will happen if you reduce the magnitude of the velocity of the green truck to a magnitude smaller than that of the red truck. Part C Click "Reset." Now, predict what will happen if you change the velocity of the red truck so that it has a magnitude smaller than that of the green truck but is negative.

A. The trucks will stick together after the collision and not move. B. The trucks will stick together after the collision and move to the left. C. The trucks will stick together after the collision and move to the right.

A. How much work is done when you push a crate horizontally with 140 N across a 8.0-m factory floor? B. If the force of friction on the crate is a steady 70 N, find the KE gained by the crate. C. Find the energy turned into heat.

A. W = 1100 J B. K = 560 J C. Q = 60 J

Part A Imagine that you replace the block in the video with a happy or sad ball identical to the one used as a pendulum, so that the sad ball strikes a sad ball and the happy ball strikes a happy ball. The target balls are free to move, and all the balls have the same mass. In the collision between the sad balls, how much of the balls' kinetic energy is dissipated? B. Now, consider the collision between two happy balls described in Part A. How much of the balls' kinetic energy is dissipated?

A.Half of it B. None of it

Why is it easier to stop a lightly loaded truck than a heavier one that has equal speed?

Because the lightly loaded truck has it has less kinetic energy, so it needs less work to stop. While the heavier truck hs more kinetic energy and therefore needs more strength to stop.

Suppose our experimenter repeats his experiment on a planet more massive than Earth, where the acceleration due to gravity is g=30 m/s2. When he releases the ball from chin height without giving it a push, how will the ball's behavior differ from its behavior on Earth? Ignore friction and air resistance. (Select all that apply.)

It will take less time to return to the point from which it was released.

How much impulse stops a 52-kg carton sliding at 4.8 m/s when it meets a rough surface?

J = 250 N⋅s

Calculate the kinetic energy of an 82-kg scooter moving at 18 m/s .

KE = 1.3×10^4 J

In which is momentum conserved: an elastic collision or an inelastic collision?

Momentum is conserved in elastic and inelastic collisions.

In terms of impulse and momentum, why do air bags in cars reduce the chances of injury in accidents?

Naturally, airbags will increase the time the force is applied and decrease the force, so the likelihood of injuring the individual decrease since it is a lesser force for a longer duration.

If a golf ball and a Ping-Pong ball both move with the same kinetic energy, can you say which has the greater speed? Explain in terms of the definition of KE. Similarly, in a gaseous mixture of heavy molecules and light molecules with the same average KE, can you say which have the greater speed?

Since the KEs are the same but masses different, then the ball with smaller mass has the greater speed. So 1⁄2Mv^2 = 1⁄2mV^2. Along with molecules, since lighter ones move faster on the average than large ones.

Does the International Space Station have gravitational PE? KE? Explain.

The International Space Station has both kinetic and potential energy because, relative to the earth, work was done to lift it in Earth's gravitational field and to impart speed to it. Therefore, both are being used.

Which exerts the greater impulse on a steel plate- machine gun bullets that bounce from the plate, or the same bullets squashing and sticking to the plate?

The bouncing bullets.

Two cars are raised to the same elevation on service-station lifts. If one car is twice as massive as the other, compare their gains of potential energy.

The car with twice the mass has twice the gain of potential energy.

If a ball is projected upward from the ground with 10 kg • m/s of momentum, what is Earth's momentum of recoil? Why do we not feel this?

The earth momentum recoil is 10 kg*m/s. Also, no we would not feel the earth because it is so huge that the recoil velocity is impossible to feel.

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?

The law of momentum conservation states that when there are no external forces acting on the system, momentum will not change at all. Since gravity (an external force) affects the ball, it is not a violation of the law when the ball gains momentum.

Select the correct equations that show that 9.6 J of work is done when a force of 8.0 N moves a book 1.2 m.

W=F⋅d=(8.0N)⋅(1.2m)=9.6J

If two objects have equal kinetic energies, do they necessarily have the same momentum? Defend your answer.

When two objects of equal kinetic energies that do not necessarily mean they have the same momentum. Even if two things happen to have the same kinetic energy, they could have different masses/velocities, which would give them different momentums since momentum=mass velocity and kinetic energy=1/2 mass velocity^2.

Part A Rank speed from greatest to least at each point. Part B Rank KE from greatest to least at each point. Part C Rank PE from greatest to least at each point.

a. DBCEA b. DBCEA c. AECBD

Suppose we replace both hover pucks with pucks that are the same size as the originals but twice as massive. Otherwise, we keep the experiment the same. Compared to the pucks in the video, this pair of pucks will rotate

at the same rate.

When traveling twice as fast your kinetic energy is increased _______.

by a factor or four.

Immediately after lunch, for each case, rank from greatest to least the speed of the formerly hungry fish.

c,a,d

Energy cannot be _______.

destroyed.

A boxer rides with the punch so as to reduce _______.

force

The work that is done when twice the load is lifted twice the distance is _______.

four times as much.

When you lift twice the load twice as high, in half the time, the increase in potential energy is _______.

four times.

Cassy can get more force on the bricks she breaks with a blow of her bare hand when _______.

her hand is made to bounce from the bricks.

A slowly moving ship can have a greater momentum than a fast-moving racing car when _______.

its mass times velocity is greater than that of the car.

What is the momentum of an 9.0-kg bowling ball rolling at 2.6 m/s ?

p = 23(kg ⋅ m)/s

What impulse occurs when an average force of 9.0 N is exerted on a cart for 2.8 s ?

p = 25( kg⋅m)/s

The quantity that is called impulse can be measured by the _______.

product of force and time.

When a big fish swims into an oncoming smaller fish and swallows it, the momentum of the two-fish system ______

remains the same.

Fossil fuels, hydroelectric power, and wind power ultimately get their energy from _______.

the Sun.

When one does twice the work in twice the time, the power expended is _______.

the same.

A 6.0-kg fish swimming 0.60 m/s swallows an absentminded 1.0-kg fish swimming toward it at a speed that brings both fish to a halt immediately after lunch. Find the speed of the approaching smaller fish before lunch.

v = 3.6 m/s

Emily holds a banana of mass m over the edge of a bridge of height h. She drops the banana and it falls to the river below. Part A Part complete Use conservation of energy to find the speed of the banana just before hitting the water. Express your answer in terms of some or all of the variables h and m, and appropriate constants.

v = √2gh

When two vehicles collide, momentum is conserved _______.

whether the collision is elastic or inelastic.