Potential energy

• 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.

• 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.

• Gravitational potential energy depends on both mass and height.

• Gravitational Potential Energy Equation

grav. PE = mass ´ free-fall acceleration ´ height

PE = mgh

• 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.

EXAMPLE - 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?

List the given and unknown values.

Given: mass, m = 65 kg

height, h = 35 m

free-fall acceleration, g = 9.8 m/s 2

Unknown: gravitational potential energy, PE = ? J

Write the equation for gravitational potential energy.

PE = mgh

Insert the known values into the equation, and solve

PE = (65 kg)(9.8 m/s 2)(35 m)

PE = 2.2 ´ 10 4 kg•m 2/s 2

PE = 2.2 ´ 10 4 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

KE = ½(mass)*speed 2

*** Kinetic energy depends on speed more than mass***

EXAMPLE- Kinetic Energy

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

List the given and unknown values.

Given: mass, m = 44 kg

speed, v = 31 m/s

Unknown: kinetic energy, KE = ? J

Write the equation for kinetic energy

KE = ½(mass)*speed 2

Insert the known values into the equation, and solve.

KE = ½(44kg)*(31m/s) 2 = 2.1 X 10 4 J