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Chapter 9
Energy
The Big Idea
Energy can change from
one form to another
without a net loss or gain.
Discussion
What is energy?
 Pros and cons?
 How many different forms of energy can
you think of?
 Take 1 minute with an elbow partner and
write down as many things you know
about energy




Energy may be the most familiar concept in
science, yet it is one of the most difficult to
define.
Persons, places, and things have energy, but
we observe only the effects of energy when
something is happening.
Only when energy is being transferred from one
place to another or transformed from one form
to another.
Work!

work = net force X distance

When you apply a force on an object and
it moves, you’ve done work.
Units of Work

Remember Work = Force x Distance.

What are the units for work?

Newton x meters

1 Nm = 1 Joule (J)
Power

Power equals the amount of work done
divided by the time interval during which
the work is done.
Units for Power

Remember
Work is measured in Joules
 Power is measured in


Joules per Second
Mechanical Energy
The energy due to the position of
something Or the movement of
something.
 The two forms of mechanical energy are
kinetic energy and potential energy.

Potential Energy

3 kinds

Elastic Potential Energy
• A stretched or compressed spring
• A stretched rubber band
• A bow in a bow and arrow

Chemical Energy
• Is released when a chemical change takes place.

Gravitational Potential Energy
• Energy of position in a gravitational field.
Gravitational Potential Energy
Focus on this type of potential energy.
 gravitational potential energy
= weight X height
Or

PE=mgh
Kinetic Energy

Energy of motion

Kinetic energy = 1/2 mass X velocity squared
Or
Total Mechanical Energy

the total Mechanical energy

Add the Kinetic and potential energy
ME = KE + PE
 Let’s think about the units.

Try it.
An object has a mass of 10kg.
 It is 5m off the ground.
 It is moving 2m/sec.

Find its gravitational potential energy.
 Find its kinetic energy
 Find its total mechanical energy

Simple Machines
A machine transfers energy from one
place to another or transforms it from one
form to another.
 A lever is a simple machine made of a
bar that turns about a fixed point.
 The fulcrum is the fixed point that a lever
operates on.

Mechanical Advantage.
work input = work output
 The ratio of output force
to input force for a machine
is called the
mechanical advantage.

Try it!

Find the mechanical advantage.
Conservation of Energy

Energy can neither be created nor
destroyed, it is only changed from one
form to another.
Work Energy Theorem
Anytime Work is done energy changes.
 We are going to think of this in terms of
kinetic energy.
 Work = KE (Kinetic Energy)
