Potential Energy Notes

Kinetic and Potential
Energy Notes
Chapter 5-2
Mechanical Kinetic Energy
• Amount of energy an object has based on
• Depends on the mass of the object and its
• Kinetic Energy = ½ mass x velocity2
– KE = ½ mv2
• Units of Energy– (kg) (m/s)2 or kg m2 / s2
– Same as unit for work—Joules (J)
• A 1400 kg car moving at a velocity of 18 m/s
has what amount of Kinetic Energy?
• How fast would a basketball with a mass of 2.3
kg have to go to have the same amount of
kinetic energy as the above car?
Work – Kinetic Energy Theorem
• The amount of work done on an object can be
translated into movement of the object
• Work can be related to the change in the
kinetic energy of the object
– Called the Work-Kinetic Energy Theorem
– Work = ∆ Kinetic Energy
• W = ∆ KE
Potential energy is stored energy
--the energy is not being used now but could
be used in the future
--2 types:
gravitational potential energy (PEg)
elastic potential energy (PEe)
Gravitational potential energy
• the energy an object has based on position
– depends on its mass, gravity, and its height above
a reference point
– gravity is trying to pull the object back down to
the earth
• Ex: apple on a tree, skier atop a hill, rock on cliff, etc.
Gravitational potential energy formula
Potential energy = mass x gravity x height
PEg = mgh
(F x d where F = mg and d = h)
--units of Joules (kg x m/s2 x m)
– Height is measured from a reference point
• Usually the ground or the surface beneath the object
• Can lead to a negative height
– Ex. An object under water or below ground, any
object below the reference point
Sample problems
• How much potential energy does a 55 g apple
have hanging 3.5 meters high in a tree?
• A 55 kg skateboarder is at the top of a ramp
with a slope of 35 degrees. The ramp is 5.5 m
long. How much potential energy does the
person have?
Elastic Potential Energy
--the amount of energy in a stretched or
compressed object
--a stretched rubber band, a stretched spring,
a compressed golf ball
• the energy comes from the molecules
wanting to return to their “normal” state
– in stretching or compressing, the molecules get
pushed closer to or pulled further from their
neighboring molecules.
– the energy comes from the attraction or repulsion
of the molecules to return to their “relaxed”
• amount of energy is based on the strength of
the spring or stretched object and the
distance it is stretched
• the spring constant (k) measures the strength
of the spring or elastic object
– the stronger the spring, the higher the k value
– the farther the spring or elastic object is
stretched, the greater strain on the
moleculesthe more potential energy
Elastic potential energy formula
• potential energy = ½ x spring constant (k) x
distance from relaxed state2
PEe = ½ k x2
--the spring constant has units of (N/m) so the units
become (N/m) x m2 which reduces to N x m or
Joules (J)
Sample elastic potential problems
• A person jumping on a pogo stick compresses
a spring with a spring constant of 9800 N/m a
distance of 15 cm. How much energy is stored
in the spring?
• A slingshot has a normal length of 26.0 cm. A
person puts a marble in it and pulls it back to
a length of 54.5 cm. The spring constant is
350 N/m. Find the energy of the rubber band.