Download Kinetic and Potential Energy

Kinetic and
Potential
Energy
Unit 7, Lesson 6
What is Energy?
What does it mean if you
have a lot of energy?
For you it means you can
run around, lift weights and
scream.
If you have energy you can
do things.
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What is Energy?
Energy is the ability to do work
This is similar to our every day definition of
energy. You can’t run up the stairs (work)
without energy!
We have lots of types of energy. Can you
name a few?
Energy comes in many different
forms
•
•
•
•
•
•
Kinetic Energy
Gravitational potential energy
Thermal Energy (heat)
Nuclear Energy (nuclear power, atomic bomb)
Electrical Energy (electricity)
Chemical Energy (food, fire, …)
We are going to look at the first two types of energy
The energy of motion
• The faster an object moves,
the more kinetic energy it has
• If two objects travel at the
same speed, the one with
more mass has more kinetic
energy
**kinetic energy depends on
speed AND mass
Image the car and bike
are moving at the
same speed.
The car has more
kinetic energy because
it has more mass
Kinetic Energy
Kinetic energy is the energy of motion
KE = ½ mv2
m = mass (kilograms)
v = velocity/speed (meters per second)
Units: Joules (J)
Let’s Practice
What is the kinetic energy of
this car when it is traveling
38.0 m/s?
What other information do
you need?
KE = ½ m v2
= ½ (1550)(38.0)2
= 1,119,100 J
Car’s mass =
1550kg
Potential Energy
Potential energy is energy that is
stored and has the potential to do
something
– There are three types of P.E.
• Elastic – Rubber band pulled
back
• Gravitational – Raising an
object off of the ground
(height & mass)
• Chemical – A log being
placed on a fire
Gravitational
Potential Energy
PE = mgh
m = mass
g = acceleration due
to gravity
h = height
Let’s Practice
A 30 kg child climbs 15 meters up a tree. When he
stops to have a look around, what is the
child’s potential energy?
GPE = mgh
First we identify the information provided in the
problem:
• mass = 30 kg
*gravity= 9.8m/sec2
• height = 15 meters
• GPE = 30kg * 9.8m/sec2 * 15 meters
= 4410 J
What’s so important about PE and KE?
We call the sum of PE and KE mechanical
energy.
ME = KE + PE
Mechanical energy is important because it is
conserved (as long as there are no non
conservative forces, like friction)
Therefore, if one goes down, the other goes up by
the same amount.
Mechanical Energy Example
A penny is dropped off the Eiffel
tower (ignore air resistance). As
it falls, what happens to it’s
potential energy? What happens
to it’s kinetic energy?
As it falls, its velocity goes up, so its
kinetic energy goes up. It also
looses height so its potential
energy goes down.
However, mechanical energy stays
the same ME = KE + PE
Practice 1
Copyright © 2004 California Department of Education.
Practice 2
Copyright © 2004 California Department of Education.
Practice 3
Copyright © 2004 California Department of Education.