Energy Test (Physics)

An object hits a wall and bounces back with half of its original speed. What is the ratio of the final kinetic energy to the initial kinetic energy?

1/4

A child and sled with a combined mass of 46.0 kg slide down a frictionless hill that is 7.04 m high. If the sled starts from rest, what is its speed at the bottom of the hill?

11.86 m/s (Set gravitational potential equal to kinetic energy. mgh = 1/2mv^2 and solve for v)

What is the kinetic energy of an automobile with a mass of 1170 kg travelling at a speed of 14.6 m/s?

124698.6 J

Car J moves twice as fast as car K, and car J has half the mass of car K. The kinetic energy of car J, compared to car K is

2:1

Each of the following objects possesses energy. Which forms of energy are mechanical, which are nonmechanical, and which are a combination? A.) glowing embers in a campfire B.) a strong wind C.) a swinging pendulum D.) a person sitting on a mattress E.) a rocket being launched into space

A.) nonmechanical (it's thermal so nonmechanial) B.) mechanical (it's moving air so It's kinetic) C.) mechanical D.) mechanical (a person on a mattress compresses the mattress springs, giving them elastic potential which is a form of mechanical energy) E.) both ( a rocket has thermal energy, light energy, sound energy (all nonmechanical) in addition to kinetic and potential)

State the principle of conservation of mechanical energy for conservative forces.

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

How can negative work be done?

It can be done on an object when the force acting on an object is opposite to the direction of motion. - ex. Friction slowing something down

What does work have to do with?

It deals with what is accomplished when a force acts on an object that moves a certain amount.

Which of the following is true of the kinetic energy of an object?

It is directly proportional to the square of its speed.

You slam on the brakes of your car in a panic, and skid a certain distance on a straight, level road. If you had been traveling twice as fast, what distance would the car have skidded, under the same conditions?

It would have skidded 4 times farther.

As a falling object's gravitational potential energy decreases, what happens?

Its kinetic energy increases.

What is the unit for kinetic energy?

Joule (J)

An acorn falls from a tree. Compare its kinetic energy K, to its potential energy U.

K increases and U decreases

What is the kinetic energy of a particle of mass (m) moving at speed (v)?

KE = 1/2 mv^2

What happens if a force is at a right angle?

No work is done by that force.

Can work be done on a system if there is no motion?

No, because of the way work is defined

A truck weighs twice as much as a car, and is moving at twice the speed of the car. Which statement is true about the truck's kinetic energy compared to that of the car?

The truck has 8 times the kinetic energy of the car

It takes no work to hold a cheerleader in the air. If no work is done by the cheerleaders, why do they eventually tire?

Their bodies expend chemical energy as their muscles function; this is "hard work", but not the same kind of work that moves a mass over a distance.

Is it possible for a system to have negative potential energy?

Yes, since the choice of the zero of potential energy is arbitrary.

Describe the energy of a car driving up a hill.

both kinetic and potential

The total mechanical energy of a system

is constant, only if conservative forces act.

If the net work done on an object is zero, then the object's kinetic energy

remains the same

What is the unit of work?

the joule

A 15 N block lifted straight upward by a hand applying a force of 15 N has an initial kinetic energy of 8 J. (A.)If the block is lifted 1 m, how much work does the hand do? (B.) what is the block's final kinetic energy?

(A.) 15 J (B.) 8 J (because the net work is zero, the net force is zero. Net work is change in kinetic energy. Since net work is zero, the change in KE is zero. Therefore it is still 8 J)

A person lifts a 3.3 kg cement block a vertical distance of 1.09 m and then carries the block horizontally a distance of 11.40 m. (A.) how much work is done by the person in the process? (B.) how much work is done by gravity in the process?

(A.) 35.97 J (B.) -35.97 J (The vertical part is the only work done. It is mgh again. Ignore the horizontal motion because it is at a right angle and therefore does no work.)

Answer the following for the situation of a punter kicking a football. (A.) Is the punter doing work on the ball while his toe is in contact with the ball? (B.) Is he doing work on the ball after the ball loses contact with his toe? (C.) what forces are doing work on the ball while it is in flight?

(A.) yes (B.) no (C.) gravity and air

A 0.5 kg air-hockey puck is initially at rest. What will it's kinetic energy be after a net force of 0.7 N acts on it for a distance of 1.2 m?

.84 J (Find the net work, which is the force times distance. This is also the change in KE. Since it is at rest, the change in KE is the sane as the KE.)

A 40 kg girl on rollerblades is swung through one revolution in a horizontal circle at the end of a 5.0 m rope held by a stationary boy at the center of the circle. The rope has a tension of 50 N in it.

0 J

A 50-N object was lifted 2.0 m vertically and is being held there. How much work is being done in holding the box in this position?

0 J

A woman with a mass of 45 kg climbs a set of stairs that are h = 3 m high. How much gravitational potential energy goes she gain?

1,359 J

Two small toys, one with a mass of 3.2 kg and the other with a velocity of 2.4 m/s, each have the same kinetic energy of 16 J. Determine the velocity of the first toy and the mass of the second.

1st: 16 = .5 (3.2) v^2 16 = 1.6v^2 10 = v^2 3.2 = v 3.2 m/s 2nd: 16 = .5 m (2.4)^2 16 = .5 m (5.76) 16 = 2.88 m 5.6 kg

When catching a baseball, a catcher's glove moves by 9 cm along the line of motion of the ball. If the baseball exerts a force of 426 N on the glove, how much work is done by the ball?

38.34 J (Work is force times distance. W = 426 x 0.09 m)

A radio-controlled car increases its kinetic energy from 6 J to 10 J over a distance of 1 m. What was the average net force on the car during this interval?

4 N (Net work is the work done by the net force. Net work is also change in KE, which is 10-6=4 J. Set 4 = to F(1 m) so F has to be 4 N.)

What is the kinetic energy is an 87 kg sprinter running at 10 m/s?

4,350 J

What is the kinetic energy of a 1450 kg sports car travelling down the road with a speed of 28 m/s?

568,400 J (Plug into the equation KE = 1/2mv^2)

If a 0.2 kg ball is dropped from a height of 3 m, what is its kinetic energy when it hits the ground?

6 J

A toy car has a kinetic energy of 12 J. What is its kinetic energy after a frictional force of 0.5 N has acted on it for 6 m?

9 J

Compared to yesterday, you did 3 times the work in one-third the time. To do so, your power output must have been

9 times yesterday's power output

What is the gravitational potential energy of a ball with a weight of 70 N when it is sitting on a shelf 1.3 m above the floor?

91 J

elastic collision

A collision in which kinetic energy is conserved

Inelastic collision

A collision in which kinetic energy is not conserved

You reach out the second story window, which is 5 m above the sidewalk, and throw a 0.5 kg ball straight upward with 6 J of kinetic energy. A.) what is the ball's gravitational potential energy when it is released? B.) what is the ball's gravitational potential energy just before hitting the side walk? C.) what is the ball's kinetic energy just before hitting the sidewalk? D.) how would the answer to part c change if the call had initially been thrown straight down with 6 J of kinetic energy?

A.) 25 J B.) 0 J (the sidewalk is 0 m. So mgh is 0 and PE is 0) C.) 31 J (add the initial PE to the 6 J you started with. Your total energy doesn't change.) D.) stays the same (answer is the same. The direction doesn't affect the kinetic energy)

Calculate the kinetic energy of the following: A.) a 2.0 g grape moving at 500 m/s B.) a 0.50 kg ball accelerated from rest by a force of 8.0 N for 3.0 m

A.) KE = .5 (0.02)(500)^2 = 250 J B.) W = (8)(3) = 24 J

If the net work done on an object is negative, then the object's kinetic energy

Decreases

A collision in which kinetic energy is conserved is called what?

Elastic

The Law of Conservation of Energy

Energy cannot be created or destroyed. Kinetic energy, however, is not always conserved. If it is not conserved, then it must have been converted into another type of energy.

In this figure, why is the kinetic energy gained by the falling ball equal to the change of potential energy? (Assuming not loss of energy due to friction)

Energy conservation requires that the total energy remains constant. In this example, the total energy is either in the form of kinetic energy of potential energy.

The force of gravity does NOT do work on an object that is what?

Stationary

What units are used for the work formula?

The F (force applied to the object) is given in Newtons, and the d (the displacement) is given in meters

What is kinetic energy?

The energy an object has because of its motion.

The quantity mgh is

The gravitational potential energy of the object.

In a rollercoaster, provided losses of thermal energy because of friction can be neglected, which of the following is true?

The kinetic energy decreases as the car moves to the top of the slope.

The quantity 1/2mv^2 is

The kinetic energy of the object

What does work depend on?

The magnitude of the force applied and on the displacement the object undergoes as a result of the application of the force.

State the work-energy principle.

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

Work-Energy Principle

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

When a force acts on an object from an angle other than 0 or 180 to the direction of motion...

The parallel vector component of the force does work but the perpendicular one does not. (Forces are right angles to the direction of motion do no work)

The work that a force does by acting on an object is equal to what?

The product of force (in the direction of motion) and distance and is positive if the force and motion are in the same direction. (that force times distance over which it acts is the work done)

State the 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.

If you hear a tree fall, what do you know?

The tree hitting the ground was an inelastic collision.

A lightweight object and a very heavy object are sliding with equal speeds along a level frictionless surface. They both slide up the same frictionless hill. Which rises to a greater height?

They both slide to the same height.

What is true about frictional forces?

They can decrease kinetic energy.

If kinetic energy is conserved by the apparatus shown in this figure, why do we find in class demonstrations that the balls eventually come to rest?

Total energy, not kinetic energy, is conserved, the rest of the energy going to heat; but the heat loss in this experiment is normally ignored since it occurs very slowly.

A girl pushes a skateboard forward with a thrust of 120 N against a frictional force from the sidewalk of 40 N while moving it 2.5 m.

W = 120 N x 2.5 m = 300 J

Lifting a 12kg crate from the floor to a platform 3.0 m above floor level.

W = 120 N x 3 m = 360 J

A gardener pushes the handle of a lawnmower with an applied force of 141 N, while pushing the mower 8.5 m.

W = 141 N x 8.5 m = 1,200 J

You throw a ball straight up. Compare the sign of the work done by gravity while the ball goes up with the sign of the work done by gravity while it goes down.

Work is - on the way up and + on the way down

A 4.0-kg mass is moving with speed 2.0 m/s. A 1.0-kg mass is moving with speed 4.0 m/s. Both objects encounter the same constant braking force, and are brought to rest. Which object travels the greater distance before stopping?

both travel the same distance

How is work calculated?

force x distance - work can be taken as the product of the applied force and the component of displacement in the direction of this force.

An object is released from rest a height h above the ground. A second object with four times the mass of the first if released from the same height. The potential energy of the second object compared to the first is

four times as much

If the net work done on an object is positive, then the object's kinetic energy

increases

A container of water is lifted vertically 3.0 m then returned to its original position. If the total weight is 30 N, how much work was done?

no work was done

Does the centripetal force acting on an object do work on the object?

no, because the force and the displacement of the object are perpendicular

The quantity Fd/t is

power

If you push twice as hard against a stationary brick wall, the amount of work you do...

remains constant at zero

Consider two masses m1 and m2 at the top of two frictionless inclined planes. Both masses start from rest at the same height. However, the plane on which m1 sits is at an angle of 30° with the horizontal, while the plane on which m2 sits is at 60°. If the masses are released, which is going faster at the bottom of its plane?

they both are going the same speed

How high will the pendulum bob swing in this figure?

to the maximum height of the bob without the pencil, as shown by the left-most position in the figure

If you walk 5.0 m horizontally forward at a constant velocity carrying a 10-N object, the amount of work you do is

zero