Giancoli AP Physics - Chapter 6

Principle of conservation of mechanical energy

If only conservative forces are acting, the total mechanical energy of a system neither increases nor decreases in any process. It is conserved.

Power

P = work/time. Measured in Watts

Joule

1 J = 1 N * m

Net work

Wsubnet = ΔKE

Is it possible for the gravitational potential energy of an object to be negative?

Yes

Watt

1 W = 1 J/s

A car accelerates from rest to 30km/h. Later, on a highway it accelerates from 30km/h to 60km/h. Which takes more energy, going from 0 to 30, or from 30 to 60?

30 to 60

When the speed of your car is doubled, by what factor does its kinetic energy increase?

4 times

A car traveling at a velocity v can stop in a minimum distance d. What would be the car's minimum stopping distance if it were traveling at a velocity of 2v?

4d. Because F*d= 1/2mv^2 2^2= 4

A cart starting from rest rolls down a hill and at the bottom has a speed of 4 m/s. If the cart were given an initial push, so its initial speed at the top of the hill was 3 m/s, what would be its speed at the bottom?

5 m/s

Hooke's law

AKA spring equation

Mike applied 10 N of force over 3 m in 10 seconds. Joe applied the same force over the same distance in 1 minute. Who did more work?

Both did the same work

Elastic potential energy

Elastic PE = 1/2 k * x^2

When you pay the electric company by the kilowatt-hour, what are you actually paying for?

Energy

Total mechanical energy

Esub2 = Esub1

A ball is thrown straight up. At what point does the ball have the most energy? Ignore air resistance.

Everywhere; the energy of the ball is the same at all of these points. Total energy is conserved if only conservative forces act. KE+PE is constant

Dissipative forces

Forces that dissipate mechanical energy rather then the total energy.

Nonconservative forces

Forces that its work depends on the path. Eg: friction.

Conservative forces

Forces which the work does not depend on the path taken rather then the initial and final positions. Eg: gravity.

A box is being pulled across a rough floor at a constant speed. What can you say about the work done by friction?

Friction does negative work

Spring equation

Fsubs = -k * x

Mike performed 5 J of work in 10 secs. Joe did 3 J of work in 5 secs. Who produced the greater power?

Joe produced more power

Kinetic energy

KE = 1/2 m * v^2

Conserved quantity

Law of conservation, Esub2 = Esub1, KE + PE = KE + PE

A bowling ball is dropped from a height h onto the center of a trampoline, which launches the ball back up into the air. How high will the ball rise?

No more than h-probably a little less.

A ball tied to a string is being whirled around in a circle. What can you say about the work done by tension?

No work is done because the force acts in a perpendicular direction to the displacement.

A satellite in circular orbit around the Earth moves at constant speed. This orbit is maintained by the force of gravity between the earth and the satellite, yet no work is done on the satellite. How is this possible?

No work is done if the direction of motion is perpendicular to the force.

You push very hard on a heavy desk, trying to move it. You do work on the desk;

Only if it starts moving. (displacement)

Potential energy

PE = m * g * h

Gravitational potential energy

PEsubgravity = m * g * y

Car #1 has twice the mass of car #2, but they both have the same kinetic energy. How do their speeds compare?

Squareroot of 2v1 = v2

Energy

The ability to do work.

In a baseball game, the catcher stops a 90-mph pitch. What can you say about the work done by the catcher on the ball?

The catcher has done negative work

You push a heavy crate down a ramp at a constant velocity. Only four forces act on the crate. Which force does the greatest magnitude of work on the crate?

The force of gravity

A man pushes a block up an incline at a constant speed. As the block moves up the incline.

The kinetic energy depends on the speed and not the position of the block. Since the block moves with constant speed, the kinetic energy remains constant. As the block moves up the incline its elevation increases, so its potential energy also increases.

Work-energy principle

The net work done on an object is equal to the change in the object's kinetic energy.

A ball is thrown straight up. Neglecting air resistance, what is not true regarding the energy of the ball?

The potential energy decreases while the ball is going up.

Horsepower is what?

The rate at which the engine can do work

In the previous question, just before hitting the ground, what is the final speed of the heavy stone compared to the light one

The same

Two paths lead to the top of a big hill. One is steep and direct, while the other is twice as long but less steep. How much more potential energy would you gain if you take the longer path?

The same

Law of conservation of energy

The total energy is neither increased nor decreased in any process. Energy can be transformed from one form to another, and transferred from one object to another, but the total amount remains constant.

Two balls are thrown off a building with the same speed on straight up and on at a 45 degree angle. What is true if air resistance can be ignored?

They both hit the ground with the same speed

Work

W = F * d. Measured in joules.

Which contributes more to the cost of your electric bill each month, a 1500-Watt hair dryer or a 600-Watt microwave oven?

depends upon how long each one is on

By what factor does the kinetic energy of a car change when its speed is tripled?

factor of 9

Two stones, one twice the mass of the other, are dropped from a cliff. Just before hitting the ground, what is the kinetic energy of the heavy stone compared to the light one?

twice as much