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In an avalanche, a mass of loose snow, soil, or rock suddenly gives way and slides down the side of a mountain.

The avalanche releases a great amount of energy.

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How are energy and work related?

Energy is the ability to do work.

Energy and Work

Work is a transfer of energy.

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Work and energy are closely related.

    • Energy is known by the changes it causes.
    • Work is done when a force moves an object through a distance. Energy is transferred by a force moving an object through a distance.
    • Both work and energy are typically measured in joules (J).

Energy and Work

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Energy has different forms.

    • The sun gives off energy in the form of heat and light.

Energy and Work

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Energy has different forms.

    • The sun gives off energy in the form of heat and light.
    • Plants convert sunlight into food.

Energy and Work

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Energy has different forms.

    • The sun gives off energy in the form of heat and light.
    • Plants convert sunlight into food.
    • People convert food energy into muscle movement.

Energy and Work

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What factors does the kinetic energy of an object depend on?

The energy of motion is called kinetic energy.

Kinetic Energy

The kinetic energy of any moving object depends upon its mass and speed.

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  • Doubling the mass in the formula doubles the kinetic energy.
  • Doubling the speed quadruples the kinetic energy.

Kinetic Energy

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Calculating Kinetic Energy

A 0.10-kilogram bird is flying at a constant speed of 8.0 m/s. What is the bird’s kinetic energy?

Kinetic Energy

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Read and Understand

What information are you given?

​

​

​

​

What unknown are you trying to calculate?

Kinetic Energy

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Read and Understand

What information are you given?

​

​

​

​

What unknown are you trying to calculate?

Kinetic Energy

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Plan and Solve

What equation contains the given quantities and the unknown?

​

​

Substitute the known values in the formula for KE.

Kinetic Energy

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Plan and Solve

What equation contains the given quantities and the unknown?

​

​

Substitute the known values in the formula for KE.

Kinetic Energy

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Look Back and Check

Is your answer reasonable?�

Kinetic Energy

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Look Back and Check

Is your answer reasonable?�

It seems reasonable, because the bird has a low mass, so it would not have much kinetic energy.

Kinetic Energy

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1. A 70.0-kilogram man is walking at a speed of 2.0 m/s. What is his kinetic energy? ��Answer: ��

Kinetic Energy

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1. A 70.0-kilogram man is walking at a speed of 2.0 m/s. What is his kinetic energy? ��Answer: ��KE = (0.50)(70.0 kg)(2.0 m/s)2 = 140 J

Kinetic Energy

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2. A 1400-kilogram car is moving at a speed of 25 m/s. How much kinetic energy does the car have? ��Answer: ��

Kinetic Energy

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2. A 1400-kilogram car is moving at a speed of 25 m/s. How much kinetic energy does the car have? ��Answer: ��= (0.50)(1400 kg)(25 m/s)2

= 440,000 J

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3. A 50.0-kilogram cheetah has a kinetic energy of 18,000 J. How fast is the cheetah running? (Hint: Rearrange the equation to solve for v.) ��Answer:

Kinetic Energy

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3. A 50.0-kilogram cheetah has a kinetic energy of 18,000 J. How fast is the cheetah running? (Hint: Rearrange the equation to solve for v.) ��Answer:

Kinetic Energy

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How is gravitational potential energy determined?

Potential energy is energy that is stored as a result of position or shape.

Potential Energy

An object’s gravitational potential energy depends on its mass, its height, and the acceleration due to gravity.

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When this musician pulls the string of her cello to one side, the string is stretched and gains potential energy.

Potential Energy

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The musician adds energy to the cello string by plucking it.

    • The energy stored in the stretched string is potential energy.
    • The stored energy is converted into kinetic energy when the string is released and it vibrates.

Potential Energy

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Gravitational Potential Energy

Potential energy that depends upon an object’s height is called gravitational potential energy.

This type of potential energy increases when an object is raised to a higher level.

Potential Energy

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This diver has gravitational potential energy as she stands at the end of a diving board.

She gained the potential energy by doing work—by climbing up the steps to the diving board.

Potential Energy

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  • The unit for mass is kilograms.
  • The unit for height is meters.
  • Acceleration due to gravity, g, has a value in SI units of 9.8 m/s2 on Earth.
  • The unit for gravitational potential energy is joules.

Potential Energy

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Height is measured from the ground or floor or some other reference level.

Doubling either the mass of the object or its height doubles its gravitational potential energy.

Potential Energy

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What is the potential energy relative to the water surface of a diver at the top of a 10.0-meter-high diving platform. Suppose she has a mass of 50.0 kilograms.

PE = mgh

= (50.0 kg)(9.8 m/s2)(10.0 m)

= 4900 kg•m2/s2 = 4900 J

Potential Energy

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Elastic Potential Energy

The potential energy of an object that is stretched or compressed is known as elastic potential energy.

Something that is elastic springs back to its original shape after it is stretched or compressed.

Potential Energy

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A compressed bicycle shock absorber and a wound-up toy robot both have elastic potential energy.

Potential Energy

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What are the major forms of energy?

Forms of Energy

The major forms of energy are mechanical energy, thermal energy, chemical energy, electrical energy, electromagnetic energy, and nuclear energy.

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All energy can be considered to be one of three forms:�

    • kinetic energy,
    • potential energy, and
    • energy in fields such as those produced by electromagnetic waves. �

Each of these forms of energy can be converted into other forms of energy.

Forms of Energy

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Mechanical Energy

The energy associated with the motion and position of everyday objects is mechanical energy.

Mechanical energy is the sum of an object’s potential energy and kinetic energy.

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Thermal Energy

The total potential and kinetic energy of all the microscopic particles in an object make up its thermal energy.

When an object’s atoms move faster, its thermal energy increases, and the object becomes warmer.

Forms of Energy

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This molten metal is extremely hot. It contains a great deal of thermal energy.

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Chemical Energy

Chemical energy is the energy stored in chemical bonds. �

When bonds are broken, the released energy can do work. All chemical compounds, including fuels such as coal and gasoline, store energy.

Forms of Energy

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This family is using the chemical energy of burning wood to produce thermal energy for heating marshmallows.

Forms of Energy

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Electrical Energy

Electrical energy is the energy associated with electric charges.

Electric charges can exert forces that do work.

Forms of Energy

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Electromagnetic Energy

Electromagnetic energy is a form of energy that travels through space in the form of waves.

Visible light and X-rays are examples of electromagnetic energy.

Forms of Energy

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  1. Lightning bolts transfer electric charge.
  2. Galaxies are giant structures in space that typically contain billions of stars. The stars give off enormous amounts of electromagnetic energy.

Forms of Energy

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Nuclear Energy

The nucleus of an atom is held together by strong and weak nuclear forces, which can store an enormous amount of potential energy.

    • The energy stored in atomic nuclei is known as nuclear energy.
    • Nuclear fission releases energy by splitting nuclei apart.
    • Nuclear fusion releases energy when less massive nuclei combine to form a more massive nucleus.

Forms of Energy

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  1. How are work and energy related?
    1. Energy is the rate of doing work.
    2. Work is a form of energy.
    3. Work is the transfer of energy.
    4. Energy is created by work.��

Assessment Questions

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  1. How are work and energy related?
    1. Energy is the rate of doing work.
    2. Work is a form of energy.
    3. Work is the transfer of energy.
    4. Energy is created by work.��ANS: C

Assessment Questions

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  1. A moving object with a mass of 10 kg has 320 J of kinetic energy due to its motion. How fast is the object moving?
    1. 64 m/s
    2. 32 m/s
    3. 8 m/s
    4. 10 m/s

�

Assessment Questions

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  1. A moving object with a mass of 10 kg has 320 J of kinetic energy due to its motion. How fast is the object moving?
    1. 64 m/s
    2. 32 m/s
    3. 8 m/s
    4. 10 m/s

�ANS: C

Assessment Questions

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  1. Which of these is an example of elastic potential energy?
    1. a bow prepared to release an arrow
    2. a rubber ball thrown into the air
    3. a book about to fall from a table
    4. a truck pulling a trailer��

Assessment Questions

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  1. Which of these is an example of elastic potential energy?
    1. a bow prepared to release an arrow
    2. a rubber ball thrown into the air
    3. a book about to fall from a table
    4. a truck pulling a trailer��ANS: A

Assessment Questions

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  1. A small airplane and a helicopter have identical masses. If the airplane’s altitude compared to the ground is three times that of the helicopter, how much more gravitational potential energy does the airplane have than the helicopter?
    1. 0.333 times as much
    2. 3 times as much
    3. 6 times as much
    4. 9 times as much��

Assessment Questions

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  1. A small airplane and a helicopter have identical masses. If the airplane’s altitude compared to the ground is three times that of the helicopter, how much more gravitational potential energy does the airplane have than the helicopter?
    1. 0.333 times as much
    2. 3 times as much
    3. 6 times as much
    4. 9 times as much��ANS: B

Assessment Questions

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  1. The energy stored in the bonds between atoms of a compound is called
    1. electromagnetic energy.
    2. chemical energy.
    3. atomic energy.
    4. thermal energy.��

Assessment Questions

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  1. The energy stored in the bonds between atoms of a compound is called
    1. electromagnetic energy.
    2. chemical energy.
    3. atomic energy.
    4. thermal energy.��ANS: B

Assessment Questions

15.1 Energy and Its Forms