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Describing energy
CHAPTER 4 SECTION 2
Change requires energy
Calorie is a unit of energy
Your body uses food as energy
The calorie is a unit of heat used to indicate the
amount of energy that foods will produce in the
human body
1 calorie is 4.1868 J
Energy is the ability to cause change
Energy can also be described as the ability to do
work, therefore energy can be measured with
the same units as work, or joules ( J )
As a soccer player swings his leg to kick a ball the
players leg has energy, the players leg and foot
cause change when the ball is kicked.
If the players leg does 250 J of work on the ball,
then 250 J of energy are transferred from the
leg/foot to the ball.
The leg/foot and soccer ball are a system.
A system is anything around you which you can
imagine a boundary.
Systems can be single objects, soccer ball, or the
solar system
When one system does work on a second system,
energy is transferred from the 1st to the 2nd
Different forms of energy
Mechanical
Electrical
Chemical
radiant
Energy is energy, just in different forms
Chemical energy and mechanical energy are
both energy just in different forms
Gasoline is chemical energy, rays from the sun
are energy but both are still just energy.
Kinetic energy – energy of motion
1
2
Kinetic Energy = mass x velocity squared
1
2
K.E. = mv2
Kinetic energy in joules, mass in kg, velocity m/s
Problems pg 116
Potential energy
Potential energy is energy that is stored due to
the interactions between objects
Ex: a rock at the top of a hill, fruit hanging from a
tree, objects connected by a compressed spring
or stretched rubber band
Elastic potential energy
Elastic potential energy is energy that is stored by
compressing or stretching an object
Chemical potential energy
Chemical potential energy is energy that is due
to chemical bonds
When chemical bonds are broken energy can be
released, during chemical reactions energy also
can be released, in the form of light energy,
thermal energy, or radiant energy
Gravitational potential energy
Gravitational potential energy is energy that is
due to the gravitational forces between objects.
Equation:
GPE ( J ) = mass (kg) x gravity (N/kg) x height (m)
GPE = mgh
Problems and section review Qs pg 119