What is Energy?In the chapter on matter, you learned that matter and energy is conserved. Instead of being created or destroyed, it is just changed from one form to another. The energy of the sunlight that reaches Earth is the ultimate source of most of the energy around us. Look at the illustration and identify all the types of energy involved.

What is Energy?

1. How did energy from sunlight provide the energy the girl needed to swing the bat? (Hint: What do you need to have energy?)

2. When the girl hits the ball, she exerts a force on it. Does she do work on the ball in the scientific sense of the term? Explain why.

3. After the girl hits the ball, the ball moves very fast and has energy. When the ball hits the fielder’s glove, it stops moving. Given that energy can never be destroyed but merely changes form, what happened to the energy the ball once had? (Hint: If you are the fielder, what do you hear and feel as you catch the ball?)

Energy and Work

• Energy is the ability to do work.• When you do work on an object, you transfer

energy to that object.• Whenever work is done, energy is transformed or

transferred to another system.

• Energy is measured in joules.• Because energy is a measure of the ability to do

work, energy and work are expressed in the same units.

Potential Energy

• The energy that an object has because of the position, shape, or condition of the object is called potential energy.

• Potential energy is stored energy.• Elastic potential energy is the energy stored in any type

of stretched or compressed elastic material, such as a spring or a rubber band.

• Gravitational potential energy is the energy stored in the gravitational field which exists between any two or more objects.

Potential Energy

• Elastic potential energy depends on the distance an object can be compressed or stretched.

• Elastic Potential Energy Equationelas. PE = ½ x spring constant (stretch distance)2

Es = ½ kd2 units N/m x m2 or J

• The spring constant (k) • Is determined by measuring the force needed to stretch

or compress a spring.k = Force/distance units N/m

Potential Energy

• Gravitational potential energy depends on both mass and height.

• Gravitational Potential Energy Equationgrav. PE = mass free-fall acceleration height

Epg = mgh units kg * m/s2 x m or J

• The height can be relative.• The height used in the above equation is usually measured

from the ground. • However, it can be a relative height between two points,

such as between two branches in a tree.

Math Skills

Gravitational Potential Energy A 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff?

1. List the given and unknown values.Given: mass, m = 65 kg

height, h = 35 mfree-fall acceleration, g = 9.8 m/s2

Unknown: gravitational potential energy, Epg = ? J

Math Skills

2. Write the equation for gravitational potential energy.

Epg = mgh

3. Insert the known values into the equation, and solve.

Ep = (65 kg)(9.8 m/s2)(35 m)Ep = 2.2 104 kg•m2/s2

Ep = 2.2 104 J

Kinetic Energy

• The energy of a moving object due to the object’s motion is called kinetic energy.

• Kinetic energy depends on mass and speed.• Kinetic Energy Equation

kinetic energy 1

2mass speed squared

KE 1

2mv 2

• Kinetic energy depends on speed more than mass.

units kg * m2/s2 or J

Math Skills

Kinetic Energy What is the kinetic energy of a 44 kg cheetah running at 31 m/s?

1. List the given and unknown values.Given: mass, m = 44 kg

speed, v = 31 m/s

Unknown: kinetic energy, KE = ? J

Math Skills, continued

2. Write the equation for kinetic energy.

kinetic energy 1

2mass speed squared

KE 1

2mv 2

3. Insert the known values into the equation, and solve.

KE 1

2(44 kg)(31 m/s)2

KE 2.1104 kggm2 /s2

KE 2.1104 J

Other Forms of Energy

• The amount of work an object can do because of the object’s kinetic and potential energies is called mechanical energy.

• Mechanical energy is the sum of the potential energy and the kinetic energy in a system.

• In addition to mechanical energy, most systems contain nonmechanical energy.

• Nonmechanical energy does not usually affect systems on a large scale.

Other Forms of Energy, continued

• Atoms and molecules have kinetic energy.• The kinetic energy of particles is related to heat and

temperature.

• Chemical reactions involve potential energy.• The amount of chemical energy associated with a

substance depends in part on the relative positions of the atoms it contains.

• Living things get energy from the sun.• Plants use photosynthesis to turn the energy in

sunlight into chemical energy.

Other Forms of Energy, continued

• The sun gets energy from nuclear reactions.• The sun is fueled by nuclear fusion reactions in its core.

• Electricity is a form of energy.• Electrical energy is derived from the flow of charged

particles, as in a bolt of lightning or in a wire.

• Light can carry energy across empty space.• Light energy travels from the sun to Earth across empty

space in the form of electromagnetic waves.