Physics Test 3

Power

- The amount of work done over the amount of time taken. - The unit of power is the Watt - W = J/s = N • m/s = kg • m^2/s^2

Conservation of efficiency concept?

- Work input = work output - Total energy before a collision = total energy after a collision - fD = Fd

What is the kinetic energy of an object with a mass of 10 kilograms traveling at a speed of 10 meters per second? Assume no other forces act upon the object.

500J

What is the equation of Power?

work ÷ time.

Truck drivers approaching a steep hill that they must climb often increase their speed. What good does this do, if any?

It's good because when the truck driver approaches the steep hill because he accelerates and when he accelerates he gets more energy which is good.

What is the unit for Potential Energy?

J(Joule)

Fulcrum

Joint of lever

A student riding a bicycle doubles his speed. By what factor does the kinetic energy of the student and bicycle change?

The student and the bicycle changes once the student changes his kinetic energy by 2.

Horsepower

Unit of power to measure the power of larger objects such as cars

Decide whether work is being done (using your physics definition of work) in the following situations: Two people pulling with the same amount of force on each end of a rope.

Work is not being done.

How are the physical concepts power and speed similar? Speed has units of _________________. Power is _________________ and work is _________________. Therefore, included in the units of power are the same units as speed.

a) Distance divided by time b) Work divided by time c) force multiplied by distance

Which combination of units can be used to express work?

a) J b) Joules. c) Kg x m^2/s^2

Work

- Involves force & distance - Force x distance

Lever

- Allows small force over large distance & large force over short distance. - Make the force less - Rotates on a point of support called fulcrum.

Law of conservation of energy

- Energy cannot be created or destroyed - It may be transformed from one form into another, but the total amount of energy never changes.

Potential energy

1) Energy resulting from an object's position or orientation. 2) Energy that a system has because of its configuration. 3) Stored energy held in readiness with a potential energy for doing work

Kinetic energy

1) Energy resulting from motion. 2) Energy that an object has because it is moving. 3) Work done on an object to change its velocity (energy in motion)

List 2 general categories of work and giving an example of each.

1) Force vs. Force - A bow & arrow vs. you 2) Acceleration - If you put your foot to the gas.

What happens when you throw a ball straight up in the air? The ball leaves your hand with _________________ energy it gained while your hand accelerated it from _________________. As the ball goes higher, it gains _________________ energy. However that ball slows down as it rises so its _________________ energy _________________. The increase in _________________ energy is exactly equal to the decrease in _________________ energy.

1) Kinetic 2) Rest 3) Potential 4) Kinetic 5) Decreases 6) Potential 7) Kinetic

Lists the types of energy:

1) Mechanical (Potential & Kinetic) 2) Electric 3) Chemical 4) Thermal 5) Sound 6) Light 7) Nuclear

Efficiency

1) Percentage of work put into a machine that is converted into useful work output - Output / input (usually top # is smaller)

List the forms of Mechanical energy

1) Potential Energy 2) Kinetic Energy

A person runs up several flights of stairs and is exhausted at the top. Later the same person walks up the same stairs and does not feel as tired. Ignoring air resistance, does it take more work to run up the stairs than to walk up?: a) More b) Less c) Equal What about energy? a) More b) Less c) Equal

1. c) Equal 2. a) More

A man ran on a treadmill for 1,800 seconds. At the end of his run, the treadmill indicated his energy output as 240,000J. What average power did he generate? Hint: P = w/t

133.33W

A hydraulic lift used at an automotive repair shop raises a 1000-kilogram car two meters off of the ground. What is the potential energy given to the car? Hint: PE = mgh

19600J

One horsepower =

746W

A person is using a force of 200N to push a box. How much power does the person need to push the box a distance of 4m in 10s?

80W

A 2.0-kilogram mass is moving with a speed of 3.0 m/s. What is the kinetic energy of the mass? Hint: KE = 1/2mv^2

9J

Machine

A device for multiplying forces or changing the direction of forces.

Two acceptable units for work _________________. Choose two. A) Joule b) Newton C) Watt D) Newton • Meter

A) Joule D) Newton • Meter

An inventor claims to have designed a perpetual motion machine, a device that creates its own power. Which of the following laws best explains why a perpetual motion machine cannot work? A) Law of conservation of energy B) Law of conservation of matter C) Newton's second law D) Newton's third law

A) Law of conservation of energy

There are a variety of units for power. Which of the following would be fitting units of power (though perhaps not standard)? Include all that apply. A) Watt B) Joule C) Joule/second D) HP

A) Watt

What 2 things occur when work is done?

Application of force, movement of something by that force

The motor of one car is more powerful than the motor of another car. Which of the following must be true of the more powerful motor? A) It can do work more quickly B) It can operate for a longer time. C) It can burn fuel more efficiently. D) It can store more potential energy.

B) It can operate for a longer time.

Two machines (e.g., elevators) might do identical jobs (e.g., lift 10 passengers three floors) and yet the machines might have different power outputs. Explain how this can be so.

Because one could take longer time.

Why are work and energy both measured in joules?

Because work is a form of energy.

Which example best illustrates kinetic energy? A) A light bulb that is turned on B) A car that is parked on top of a hill C) A comet that is flying through space D) A battery that is connected to a circuit

C) A comet that is flying through space

A group of students were setting up their school auditorium for a rock concert to be held over the weekend. The students used a ramp to get on of the large speakers onto the stage. The students know that work = force x distance. Which of the following best explains the advantage of using the ramp instead of lifting the speaker onto the stage? A) The mass of the speaker is reduced by using the ramp. B) More energy is created by using the ramp. C) Less force is required by using the ramp. D) The amount of work done is decreased by using the ramp.

C) Less force is required by using the ramp.

A car, a train, a bicycle, and a skateboard are traveling at the same speed. Which object has the greatest kinetic energy? A) Car B) Bicycle C) Train D) Skateboard

C) Train

Two students each carry a box up to the third floor of a building. The total mass of each student and the box she is carrying is the same. Roberta makes the trip in 45 s while Mary takes 30 s. Which of the following statements best describes this task? A) Robert does more total work than Mary. B) Mary does more total work than Roberta C) Roberta has a greater power output than Mary. D) Mary has a greater power output than Roberta.

D) Mary has a greater power output than Roberta.

A 5kg rock and a 10kg rock are dropped from a height of 10m. Which of the following statements describes what happens to the potential energy of the rocks? a) Neither rock loses potential energy because no work is done on the rocks. b) Bother rocks lose the same amount of potential energy because they fall from the same height. c) The 10kg rock loses more potential energy than the 5kg rock because the 10kg rock falls faster. d) The 10kg rock loses more potential energy than the 5kg rock because the 10kg rock has more mass.

D) The 10kg rock loses more potential energy than the 5kg rock because the 10kg rock has more mass.

Power is defined as the _________________ is done. A) Amount of work which B) Angle at which C) Direction at which work D) The rate at which work.

D) The rate at which work.

A force acting upon an object to cause a displacement is known as _________________. A) Energy B) Potential C) Kinetic D) Work

D) Work

Heat, light, and sound are all different forms of:

Energy

One joule is equivalent to one:

Force over specific distance.

A physics instructor demonstrates energy conservation by releasing a heavy pendulum bob, as shown in the sketch, allowing it to swing to and fro. What would happen if in his exuberance he gave the bob a slight shove as it left his nose? Explain.

If he gave the bob a slight shove as it left his nose, it would come back and hit his nose because by pushing the bob, the teacher gave the bob enough energy to return and hit him in the nose.

Mechanical energy

It is due to position or to motion, or both.

Watt

It is the measurement unit of power equal to one joule of work performed in 1 second

At what point in its motion is the KE of a pendulum bob a maximum? At what point is its PE a maximum? When its KE is half its maximum value, how much PE does it have?

KE is at it's maximum when it's in the middle, and it's at it's maximum PE when it's at its starting point. KE is half it's maximum value when it's in between the starting point and the middle. When the bob is in between the starting point and the middle, it is also at it's half maximum value.

A student is holding a stack of books Is the student doing any work on the books? Explain your answer.

No, because in order to do work you have to be releasing a force at a certain distance.

What is the meaning of the unit of power called a watt?

One watt is equal to one joule of work per one second of time.

What is the equation of Potential Energy?

PE = mass • gravity • height

A high diver steps off a diving platform that is 10 meters above the water. If no air resistance is present, during the fall there will be a decrease in the diver's:

Potential Energy

What is the difference between work and power?

Power is the rate at which work is done.

What is the pulling force in a rope known as?

Tension

What is the equation of Kinetic Energy?

W = (delta) KE = 1/2(m)(v^2)

What is the equation of Work?

W = Fd

What is the unit for Power?

Watt

Noah carried a skateboard up a hill and then rode the skateboard down the hill. When Noah reached the bottom of the hill, he rolled to a stop. When did Noah have the most potential energy?

When he was on the top of the hill.

Force multiplied by distance is equal to:

Work

Decide whether work is being done (using your physics definition of work) in the following situations: Hitting a tennis ball with a tennis racket.

Work is being done.

Decide whether work is being done (using your physics definition of work) in the following situations: Picking up a bowling ball off the floor.

Work is being done.

Decide whether work is being done (using your physics definition of work) in the following situations: Pushing against a book so it slides across the floor.

Work is being done.

Decide whether work is being done (using your physics definition of work) in the following situations: Pushing hard against a wall for an hour.

Work is not being done.

Decide whether work is being done (using your physics definition of work) in the following situations: Standing very still with a book balanced on your head.

Work is not being done.

Two cranes are lifting identical steel beams at the same time. One crane is putting out twice as much power as the other. Assuming friction is negligible, what can you conclude is happening to explain this difference? a) The crane with double the power must be lifting its beam four times as fast. b) The crane with double the power must be lifting its beam twice as fast. c) The crane with double the power must be lifting its beam twice as slowly. d) The crane with double the power must be lifting its beam four times as slowly.

b) The crane with double the power must be lifting its beam twice as fast.

When you throw a ball, the work you do to accelerate it equals the kinetic energy the ball gains. If you do twice as much work when throwing the ball, does it go twice as fast? Explain. a) Yes. Twice as much work will give the ball twice as much kinetic energy. Since KE is proportional to the speed, the speed will double as well. b) Yes. Twice as much work will give the ball four times as much kinetic energy. Since KE is proportional to the speed squared, the speed will be the square root of 4, or twice as fast. c) No. Twice as much work will give the ball twice as much kinetic energy. But since KE is proportional to the speed squared, the speed will be √2 times larger. d) No. Twice as much work will give the ball four times as much kinetic energy. Since KE is proportional to the speed, the speed will be four times larger.

c) No. Twice as much work will give the ball twice as much kinetic energy. But since KE is proportional to the speed squared, the speed will be √2 times larger.

A stone is dropped from a certain height and penetrates into mud. All else being equal, if it is dropped from twice the height, how much farther should it penetrate? a) Four times as far b) The same distance c) Twice as far d) Three times as far

c) Twice as far

Describe the motion of an object that possesses kinetic energy yet undergoes no net displacement. a) The object must be rolling b) The object must be sliding c) The object must be at rest d) The object must be spinning e) The object must be falling

d) The object must be spinning