What is energy?
--Ability to do work!
Forms of energy
Thermal Energy
the kinetic energy of atoms and molecules
(remember, “heat” is the transfer of this energy
between systems)
Chemical energy
energy associated with electronic structure of atoms
and the electromagnetic force
Nuclear energy
energy associated with nuclear structure of atoms
and the strong nuclear force
Electrical energy
associated with an electric current (kinetic energy
of electrons in a conductor)
Radiant (light) energy
energy associated with photons of light
Mechanical energy
associated with the movement of position of
physical bodies (kinetic and potential energy)
Energy can be categorized into two main classes:
 Kinetic energy– energy a body has
because it is moving
𝟏
𝟐
KE= 𝒎𝒗
𝟐
 Kinetic energy is a scalar quantity with unit of
Joules (Newton*Meter)
 Since it involves velocity, it depends on the
frame of reference
 Potential Energy: energy that is stored in
a body.
 Gravitational potential energy:
Energy associated with gravitational force.
That is, energy that goes into lifting an
object is stored in the object as gravitational
potential energy.
PE=mgh
𝒎
g is the acceleration due to gravity (9.81 𝟐 )
𝒔
h is the height (relative to the reference height)
It’s a scalar quantity with unit of Joules
(Newton*Meter).
Gravitational potential energy is relative
because h is relative. A reference height
has to be set first.
Change of gravitational energy doesn’t
depend on the path, only height.
(conservative forces)
Practice
A ball of mass m collides into a stationary ball with
the same mass at a velocity of v. Suppose the
collision is elastic collision.
a. What are the velocities of the two balls after the
collision?
b. What is the total kinetic energy of the system
before collision?
c. What is the total kinetic energy of the system
after collision?
d. Is the total kinetic energy conserved?
Practice
A ball of mass m collides into a stationary ball with the same
mass at a velocity of v. Suppose the collision is completely
inelastic collision.
a. What are the velocities of the two balls after the collision?
b. What is the total kinetic energy of the system before
collision?
c. What is the total kinetic energy of the system after collision?
d. Is the total kinetic energy conserved?
Momentum
conserved
Kinetic energy
conserved
Elastic
collision
Yes
Yes
Inelastic
collision
Yes
No
Completely
inelastic
collision
Yes
No
Practice
Drop a ball of 1 kg mass from a height of 10 meters.
Compute the potential energy before it is dropped and
the kinetic energy just before it hits the floor. Are they
the same?