Physical Science
Chapter 15
Energy
The Nature of Energy

Energy – the ability to do work or cause a
change.
 work is the transfer of energy
 If you lift a box, do you do work?
 Yes, and THIS requires ENERGY!!
 SI unit for energy is the same as the SI unit
for work
 Joule
Two main types of energy: Kinetic and
Potential
Kinetic Energy: the energy of motion
 Potential Energy: Energy stored for use at
a later time

Kinetic Energy (KE)





KE is the energy of motion
depends on the object’s mass and velocity
Energy is transferred during work
 The more work done, the more energy
transferred
Kinetic energy = Mass x Velocity2
2
When mass is doubled, Kinetic Energy is


Doubled
When velocity is doubled; Kinetic Energy is

quadrupled
What’s the Kinetic Energy?

What is the Kinetic Energy (in Joules) of an object with a
mass of 10 kg and a velocity of 10 m/s?

When mass is doubled; Kinetic Energy is doubled

When velocity is doubled; Kinetic Energy is quadrupled!!
Kinetic Energy Problems

A 70.0 kg man is walking at a speed of 2.0 m/s.
What is his kinetic energy?
Kinetic Energy Problems

A 1400 kg car is moving at a speed of 25 m/s.
How much kinetic energy does the car have?
Potential Energy:

Energy stored for use at a later time
Elastic Potential Energy



Energy of an object that is stretched or
compressed
Ex. Rubber band, springs, bouncing ball
Human slingshot video clip
Gravitational Potential Energy
 Depends
on an
object’s Height
and weight
(notice it’s
weight, NOT
mass!)
 Think about
cliff diving
Gravitational Potential Energy (PEg)
 GPE
= work done to lift an object to
a height
 GPE = Weight x Height (remember
that weight = mass x 9.8 m/s2)
 GPE = mass x 9.8 m/s2 x Height
(Fgh)

What is the potential energy of a rock that
weighs 100N that is sitting on top of a hill 300
m high?

PEg = 100 N x 300 m = 30,000 Nm = 30,000 Joules
Mechanical Energy




energy associated with the motion and position
of an object
sum of an object’s potential and kinetic energy
GPE =
PE + KE = ME
Weight x
Height
NOT limited to machines
Law of Conservation of Energy


energy cannot be created or destroyed
it can be converted from one form to another
Conservation of Energy

If energy is not created or destroyed, why does a
car stop moving when you let it coast?




Energy conversions
when pool balls collide?
Energy conversions in a
pendulum's motion?
Energy conversions in
the pole vault?
Energy conversions as a
ball bounces?
Conservation of Energy
Energy Conversion Calculations

when friction is small enough to be ignored, the
mechanical energy of a closed system does not
change.

Remember: mechanical energy = KE + PE
More Practice

A 1715 N thrill-seeker goes cliff diving. If he
hits the water traveling at 343 m/s, how high
was the cliff?