Energy
Chapter 8
Key Terms
Work – The change in energy of an object
W=Fxd
 Power – The time at which you complete
work
P=W/t
 Joule – The unit of work, Newton-meter
 Watt – The unit of power, joule per second

Key Terms 2
Energy – The property of a system that
enables it to do work
 Potential Energy – The stored energy that a
body possesses because of its position
PE = mgh
 Kinetic Energy – The energy of motion
KE = ½ mv2

Work
W=Fxd

The amount of work done is only dependent
upon the force of gravity on an object and
the distance it is being raised.
 The objects of the same mass, but are being
pulled at different angles up a ramp will
have the same amount of work exerted upon
them.
Power
P = W/t

If equal loads are
lifted to the same
height, the forces and
distances are equal, so
the work is the same.
What will change is
the power, or rate at
which work is done.
Potential Energy
PE = mgh

The potential energy is only dependent to a
height, h, above a given base level, and the
weight, mg, of an object (Remember:
weight is the force due to gravity)
Kinetic Energy
KE = ½ mv2

Heavy objects will have more kinetic
energy than light ones
 Kinetic energy depends more on speed than
mass
The Conservation of Energy
Energy cannot be created or
destroyed; it may be transformed
from one form to another, but the
total amount of energy never
changes.
Conservation of Energy for
Downhill Skiing
Conservation of Energy in a
Pendulum
Conservation of Energy on a
Roller Coaster
Thought Question 3
A high diver has a potential energy of 10,000
J at the top of the diving platform.
1. What is her kinetic energy when she gets
to the bottom?
2. What is her kinetic and potential energy
when she is halfway through her dive?
Machines

A machine is a device used to multiply
forces or simply change the direction of
forces
 Every machine uses the concept of
conservation of energy
 The lever is an example of a simple
machine, one that changes the direction of
force by doing work on one end
Work input = work output
Levers
Pulleys

Another type of lever that can multiply
forces
Mechanical Advantage
Mechanical advantage = output force / input force
Mechanical advantage = input distance/output distance
Efficiency

The ratio of useful energy output to total energy
input, the percentage of the work input that is
converted to work output
Efficiency = useful work output/ total work input
Efficiency = actual mechanical advantage/
Theoretical mechanical advantage
Assignment

Read Chapter 8 (pg. 103-117)
 Do Chapter 8 Assessment #31-47 (pg. 120-121)
 Appendix F #1-21 (pg. 672-673)