What is Potential Energy?
Potential energy is energy stored in an object. This energy has the
potential to do work. Gravity gives potential energy to an object. This
potential energy is a result of gravity pulling downwards. The gravitational
constant, g, is the acceleration of an object due to gravity. This
acceleration is about 9.8 meters per second on earth. The formula for potential
energy due to gravity is PE = mgh. As the object gets closer
to the ground, its potential energy decreases while its kinetic energy
increases. The difference in potential energy is equal to the difference
in kinetic energy. After one second, if the potential energy of an object
fell ten units than its kinetic energy has risen ten units. Potential energy
units are joules.
W = Fd kg m/s2 meters Work equals force times distance,
F = ma kg m/s2 Force equals mass times acceleration,
a = g m/s2 Acceleration is g,
F = mg kg m/s2 So force equals mass
W = mgd kg m/s2 meters Which means work equals mass
times g times distance,
d = h meters Distance equals height,
W = mgh kg m/s2 meters Which makes work equal to mass
times g times height,
W=PE Which means potential energy it the amount of work an object
PE = mgh
PE = (20 kg)(9.8 m/s2)(10 m)
PE = (20 kg)(98 m2/s2)
PE = 1960 kg m2/s2 or 1960 joules
This block can perform 1960 joules of work.
- Take out a rubber band.
- Stretch out the rubber band.
- Ask the students if it has any energy.
- Release rubber band.
- Explain to the students that it did have energy, potential energy.
This energy became kinetic energy as the rubber band was released.
- Explain that potential energy is the stored energy in an object and
that it has the potential to become kinetic energy.
- Tell the students gravity causes objects above the ground to have potential
energy. If the object stays in the air, it has the potential to fall.
- Explain to the students that there are three things required to calculate
how much potential energy gravity causes an object to have.
- Tell them that the first is how high the object is off the ground,
the second is the mass of the object, and the last is the amount of gravitational
- Tell the students that potential energy is equal to mgh. In
this equation, m is mass in kilograms, g is acceleration
due to gravity in meters per second squared, and h is the height
of the object.
- Explain that gravitational acceleration is a constant, on earth it
is 9.80655 meters per second.
- Pick up some object that you can drop.
- Raise it in the air.
- Tell the students that the object now has potential energy because
- Drop the object.
- Explain to the srted into kinetic energy.
- Now have the students use their model to do the following demonstration:
- Put two characters on the platform of the model and one on the far
end of the lever.
- Slide the two characters down their wire onto the end of the lever.
- Now lift the other character off having him follow the copper wire
- Explain that the two characters had potential energy while on top of
the platform and as they jumped it turned into kinetic energy. This kinetic
energy transferred to the other side lifting the other performer into the
- Tell the students that the performers have potential energy whenever
they are in a position where they will fall.
- Explain that potential energy is used to calculate how high a performer
will be launched off a lever.
- Tell them that the change in potential energy is equal to the opposite
of the change in kinetic energy. For example, if a ball lost ten units
of potential energy it gained ten units of kinetic energy.
- Explain that if the potential energy falls to zero then the kinetic
energy rises to whatever the potential energy was before.
- Tell the students that potential energy problems also need to have
- Write the following problem on the board or overhead:
Richard wants to know how much potential energy his cat has when it
climbs to the top of the tree near his house. The tree is 15 meters high
and the cat has a mass of 5 kilograms. How much potential energy does the
- Give the students a few minutes to solve the problem.
- Then write the following solution on the board:
PE = mgh
PE = (5 kg)(9.8 m/s2)(15 meters)
PE = (5 kg)(147 m2/s2)
PE = 735 kg m2/s2 or 735 joules
- Have the students work on the two worksheets on potential energy and
finish them for homework.
What it Leads Up To
Potential energy leads up to:
- Potential Energy in springs - a different formula with different applications
- Potential energy is __________ energy.
- __________ is one thing that causes potential energy.
- An object has potential energy if it has ___________.
- ________, ________, and __________ affect the amount
of potential energy in an object.
- The formula for potential energy is ____________.
- __________ must be shown with potential energy problems.
- The change in potential energy is _________ to the opposite
of the change in kinetic energy.
- Gravitational acceleration is a ___________.
- An object potential energy at the __________ of a fall
equals an objects kinetic energy at the __________ of a fall.
- Write down one real world example of potential energy.
- John has an object suspended in the air. It has a mass
of 50 kilograms and is 50 meters above the ground. How much work would
the object do if it was dropped?
- Mrs. Jacobs dropped an object from 10 meters. She knows
it did 50 joules of work. How much did it weigh?
- mass; gravity; height
- PE = mgh
- start; end
- answers will vary
- Solution: 24516.375 joules
PE = mgh
PE of the object is (50 kg)(9.80655 m/s2)(50
PE = (2500 kg m)(9.80655 m/s2)
PE = 24516.375 kg m2/s2 or
If it was dropped it could do 24516.375 joules of work.
- Solution: .5 kilograms
PE = mgh
PE of the object is 50 joules
50 kg m2/s2 = m(9.80655 m/s2)(10
Solve for m
5 kg m/s2 = (9.80655 m/s2)m
m = .5098 kg
The mass of the object was about .5 kilograms.