# types of energy

## kqed make it at home tabletop linear accelerator

QUEST teams up with Make Magazine to construct the latest must have, do-it-yourself device hacks and science projects. This week well show you how to make a tabletop linear accelerator that demonstrates the finer points of kinetic energy by shooting a steel ball. For more information on the tabletop linear accelerator, see http://science.k MEDIA Click image to the left or use the URL below. URL:

## elastic potential energy

Potential energy due to an objects shape is called elastic potential energy. This energy results when elastic objects are stretched or compressed. Their elasticity gives them the potential to return to their original shape. For example, the rubber band in Figure 17.6 has been stretched, but it will spring back to its original shape when released. Springs like the handspring in the figure have elastic potential energy when they are compressed. What will happen when the handspring is released?

## energy conversion

Remember the diver in Figure 17.5? What happens when he jumps off the diving board? His gravitational potential energy changes to kinetic energy as he falls toward the water. However, he can regain his potential energy by getting out of the water and climbing back up to the diving board. This requires an input of kinetic energy. These changes in energy are examples of energy conversion, the process in which energy changes from one type or form to another.

## conservation of energy

The law of conservation of energy applies to energy conversions. Energy is not used up when it changes form, although some energy may be used to overcome friction, and this energy is usually given off as heat. For example, the divers kinetic energy at the bottom of his fall is the same as his potential energy when he was on the diving board, except for a small amount of heat resulting from friction with the air as he falls.

## examples of energy conversions

There are many other examples of energy conversions between potential and kinetic energy. Figure 17.7 describes how potential energy changes to kinetic energy and back again on swings and trampolines. You can see an animation of changes between potential and kinetic energy on a ramp at the URL below. Can you think of other examples?

## defining energy

The concept of energy was first introduced in the chapter "States of Matter," where it is defined as the ability to cause change in matter. Energy can also be defined as the ability to do work. Work is done whenever a force is used to move matter. When work is done, energy is transferred from one object to another. For example, when the batter in Figure 17.2 uses energy to swing the bat, she transfers energy to the bat. The moving bat, in turn, transfers energy to the ball. Like work, energy is measured in the joule (J), or newtonmeter (Nm). Energy exists in different forms, which you can read about in the lesson "Forms of Energy" later in the chapter. Some forms of energy are mechanical, electrical, and chemical energy. Most forms of energy can also be classified as kinetic or potential energy. Kinetic and potential forms of mechanical energy are the focus of this lesson. Mechanical energy is the energy of objects that are moving or have the potential to move.

## kinetic energy

What do all the photos in Figure 17.3 have in common? All of them show things that are moving. Kinetic energy is the energy of moving matter. Anything that is moving has kinetic energy from the atoms in matter to the planets in solar systems. Things with kinetic energy can do work. For example, the hammer in the photo is doing the work of pounding the nail into the board. You can see a cartoon introduction to kinetic energy and its relation to work at this URL: . The amount of kinetic energy in a moving object depends on its mass and velocity. An object with greater mass or greater velocity has more kinetic energy. The kinetic energy of a moving object can be calculated with the equation: 1 Kinetic Energy (KE) = mass velocity2 2 This equation for kinetic energy shows that velocity affects kinetic energy more than mass does. For example, if mass doubles, kinetic energy also doubles. But if velocity doubles, kinetic energy increases by a factor of four. Thats because velocity is squared in the equation. You can see for yourself how mass and velocity affect kinetic energy by working through the problems below. Problem Solving Problem: Juan has a mass of 50 kg. If he is running at a velocity of 2 m/s, how much kinetic energy does he have? Solution: Use the formula: KE = 12 mass velocity2 1 50 kg (2 m/s2 ) 2 = 100 kg m2 /s2 = 100 N m, or 100 J KE = You Try It! Problem: What is Juans kinetic energy if he runs at a velocity of 4 m/s? Problem: Juans dad has a mass of 100 kg. How much kinetic energy does he have if he runs at a velocity of 2 m/s?

## potential energy

Did you ever see a scene like the one in Figure 17.4? In many parts of the world, trees lose their leaves in autumn. The leaves turn color and then fall from the trees to the ground. As the leaves are falling, they have kinetic energy. While they are still attached to the trees they also have energy, but its not because of motion. Instead, they have stored energy, called potential energy. An object has potential energy because of its position or shape. For example leaves on trees have potential energy because they could fall due to the pull of gravity.

## gravitational potential energy

Potential energy due to the position of an object above Earth is called gravitational potential energy. Like the leaves on trees, anything that is raised up above Earths surface has the potential to fall because of gravity. You can see examples of people with gravitational potential energy in Figure 17.5. Gravitational potential energy depends on an objects weight and its height above the ground. It can be calculated with the equation: Gravitational potential energy (GPE) = weight height Consider the diver in Figure 17.5. If he weighs 70 newtons and the diving board is 5 meters above Earths surface, then his potential energy is: GPE = 70 N 5 m = 350 N m, or 350 J

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

energy stored in an object because of its position or shape

``````a. energy

b. kinetic energy

c. energy conversion

d. work

e. gravitational potential energy

f. elastic potential energy

-->  g. potential energy
``````

A leaf hanging motionless on a tree has

``````a. no energy.

b. elastic energy.

c. kinetic energy.

-->  d. potential energy.
``````

stored energy due to an objects shape

``````a. energy

b. kinetic energy

c. energy conversion

d. work

e. gravitational potential energy

-->  f. elastic potential energy

g. potential energy
``````

When a moving bat hits a ball, what happens to the kinetic energy of the bat?

``````a. All of it becomes potential energy.

-->  b. Most of it is transferred to the ball.

c. All of it is used up and gone.

d. Most of it changes to heat.
``````

use of force to move matter

``````a. energy

b. kinetic energy

c. energy conversion

-->  d. work

e. gravitational potential energy

f. elastic potential energy

g. potential energy
``````

A 40-kilogram boy is running at a velocity of 3 m/s. What is his kinetic energy?

``````-->  a. 180 J

b. 120 J

c. 43 J

d. 13 J
``````

Lana, who weighs 400 newtons, is about to dive from a 10-meter diving board. Her gravitational potential energy is

``````a. 40 J

b. 2000 J

-->  c. 4000 J

d. 40,000 J
``````

energy of moving matter

``````a. energy

-->  b. kinetic energy

c. energy conversion

d. work

e. gravitational potential energy

f. elastic potential energy

g. potential energy
``````

stored energy due to an objects position

``````a. energy

b. kinetic energy

c. energy conversion

d. work

-->  e. gravitational potential energy

f. elastic potential energy

g. potential energy
``````

Energy is converted from kinetic energy to potential energy when you

``````a. ski down a hill.

-->  b. climb a mountain.

c. run around a level track.

d. two of the above
``````

ability to do work

``````-->  a. energy

b. kinetic energy

c. energy conversion

d. work

e. gravitational potential energy

f. elastic potential energy

g. potential energy
``````

process in which energy changes from one type or form to another

``````a. energy

b. kinetic energy

-->  c. energy conversion

d. work

e. gravitational potential energy

f. elastic potential energy

g. potential energy
``````

The atoms of matter have kinetic energy.

``````-->  a. true

b. false
``````

An objects velocity affects its kinetic energy more than its mass does.

``````-->  a. true

b. false
``````

A heavier object has less gravitational potential energy than a lighter object at the same height.

``````a. true

-->  b. false
``````

Compressing a spring gives it potential energy.

``````-->  a. true

b. false
``````

Energy conversions cannot be reversed.

``````a. true

-->  b. false
``````

The ability to cause a change in matter is one definition of

``````a. work.

b. force.

-->  c. energy.

d. motion.
``````

Forms of energy include

``````a. mechanical energy.

b. electrical energy.

c. chemical energy.

-->  d. all of the above
``````

What is the kinetic energy of an object that has a mass of 10 kg and a velocity of 1 m/s?

``````a. 100 J

b. 10 J

-->  c. 5J

d. 1J
``````

What is the gravitational potential energy of an object that has a weight of 12 N and is 3 m above the ground?

``````a. 108 J

-->  b. 36 J

c. 15 J

d. 4J
``````

Which statement is false about objects with kinetic energy?

``````a. They are in motion.

b. They are doing work.

c. They are moving matter over a distance.

-->  d. They are using up their energy by moving.
``````

The SI unit for energy is the

``````a. joule.

b. newton.

c. newton  meter.

-->  d. two of the above
``````

Which type(s) of energy does a person have when jumping on a trampoline?

``````a. kinetic energy

b. elastic potential energy

c. gravitational potential energy

-->  d. all of the above
``````

Most forms of energy can also be classified as kinetic or potential energy.

``````-->  a. true

b. false
``````

If the mass of an object doubles, its kinetic energy is only half as great.

``````a. true

-->  b. false
``````

Kinetic energy and velocity have an inverse relationship.

``````a. true

-->  b. false
``````

Clothes hanging motionless on a clothesline do not have any energy.

``````a. true

-->  b. false
``````

Changing the shape of an elastic material gives it potential energy.

``````-->  a. true

b. false
``````

If you double the weight of an object, its gravitational potential energy also doubles.

``````-->  a. true

b. false
``````

The higher above the ground you are, the less gravitational potential energy you have.

``````a. true

-->  b. false
``````

The energy of a child on a swing changes back and forth between kinetic and potential energy.

``````-->  a. true

b. false
``````

Some of the kinetic energy of the child in question 8 is given off as heat.

``````-->  a. true

b. false
``````

Energy conversions are always permanent changes in energy.

``````a. true

-->  b. false
``````

## diagram questions

No diagram questions associated with this lesson
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