Download Mechanical Energy - Lancaster High School

Mechanical Energy
Ch. 4
Energy

Is the ability to do
work.

Energy = work

Units = Joules (J)
James Prescott Joule
Work
Work is a force
applied through a
distance.
 W=Force(N) x
distance (m)

◦ W=Fxd
◦ SI Unit = Joules (J)

If you apply a force,
but do not move, you
are not doing work.


Joe pushes the mower
with 250 N of force for
20 m. How much work
has he done?
W = Fxd
250 N x 20 m = 5000 J
Power




Power is the rate at
which energy is
converted.
◦ Or, it’s how fast you
do work.
Power = Work (J) / time
(s) = Watts (W)
or
Power = Energy (J) /
time (s)
SI Unit = Watts (W)
◦ Watts = Joules /
seconds
If a light bulb
produces 300 J of
energy in 5 s, how
much power does it
produce?
 P = W/t
 P = 300J / 5 s
 P = 60 watts

Potential energy
Stored energy – has the potential to do
work
 Elastic PE – energy stored by stretching
or compressing an object

◦ Ex. – rubber band, spring

Chemical PE – energy stored in chemical
bonds
◦ Ex. – food, gasoline, batteries
Potential energy
Gravitational PE – stored energy due to
gravitational attraction between objects
 Depends on mass, gravity, and height
 Formula: GPE = mass x gravity x height

◦ GPE = mgh
 g = 9.8 m/s2
 Unit = Joules (J)
If h, then GPE
 If m, then GPE

Kinetic Energy
Energy of motion – an object must be
moving to have KE
 Depends on mass and velocity
 Formula: KE = ½ mass x velocity2

◦ KE = ½ mv2
◦ Units are Joules (J)

Depends more on velocity than mass
◦ v is squared
Work Energy Theorem

Work = Change in Kinetic Energy
◦ W = Δ KE

To change the KE on an object, work
must be done on the object.
◦ The more KE an object has, the more
work must be done to change that energy.
 Car accelerating and decelerating examples
Conservation of Energy
Energy can not be created or destroyed.
It can only be transformed from one type
of energy to another.
 The total amount of energy never
changes!!!
 Pendulum Example
