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)