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Potential and Kinetic Energy
What is Energy
• Energy is the ability to do work.
• Measured in Joules (J).
• Mechanical energy is the energy due to the
position of something or the movement of
something.
• Energy is never created or destroyed, it is only
transformed into a different form.
Forms of Energy
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Chemical - gasoline
Electrical - lightning
Heat – burner on a stove
Light – flash light
Mechanical - car
Nuclear - sun
Sound – music on the radio
Mechanical Energy
• What are the two forms of mechanical energy?
Potential and Kinetic Energy
• Energy that is stored is called potential
energy (PE) because in the stored state it has
the potential for doing work.
• Three examples of potential energy are elastic
potential energy, chemical energy, and
gravitational potential energy.
Examples of Potential Energy:
A stretched rubber band..
Water at the top of a waterfall..
Yo–Yo held in your hand..
A drawn Bow and Arrow…
Potential Energy
• Work is required to elevate objects against
Earth’s gravity.
• The potential energy due to elevated positions
is gravitational potential energy (GPE).
• The higher an object, the more potential
energy.
• The more mass an object has, the more
potential energy it has.
Potential Energy
• The amount of gravitational potential energy
possessed by an elevated object is equal to the
work done against gravity to lift it.
GPE = mgh
• The height is the distance above some chosen
reference level, such as the ground or the floor.
Potential Energy
• The potential energy of the 100-N boulder with respect to the
ground below is 200 J in each case.
a.
b.
The boulder is lifted with 100 N of force.
The boulder is pushed up the 4-m incline with 50 N of force.
c.
The boulder is lifted with 100 N of force up each 0.5-m stair.
Kinetic Energy
• When stored energy begins to move, the object
now transfers from potential energy into
kinetic energy.
Kinetic Energy
• If an object is moving, it has energy. (Be
careful, the converse of this statement is not
always true!)
• This energy is called kinetic energy - the
energy of motion.
Kinetic Energy
• An object’s kinetic energy depends on:
the object’s mass.
– Kinetic energy is directly proportional to mass.
the object’s speed (velocity).
– Kinetic energy is directly proportional to the
square of the object’s speed.
Potential Energy Check
• If I want to drop an apple from the top of one of these
three things, where will be the most potential energy?
A
B
C
Since the tower is the highest off
the ground, the apple would have
the most PE (PE = mgh)
Kinetic Energy Check
When these objects move at the same speed,
which will have more kinetic energy?
The truck will have a
higher KE at the same
speed because it has a
larger mass
KE = ½ mv2
Work – Energy Theorem
• When you throw a ball, you do work on it to
give it speed as it leaves your hand. The
moving ball can then hit something and push
it, doing work on what it hits.
• Common units of kinetic energy: Joules
Work – Energy Theorem
• The work-energy theorem describes the
relationship between work and energy.
• Work = ∆KE; ΔPE = -Work
• Work equals the change in kinetic energy.
• The work in this equation is the net work—that
is, the work based on the net force.