Transcript What is Energy?

What is Energy?
Energy, work and Power
• Energy is the ability to do work or cause
change
Work and Energy
• When an object does work on another
object, some energy is transferred to the
object…
• So work = a transfer of energy
• Both are measured in Joules (J)
Power and Energy
• Power is the rate at which work
is done
• Power is therefore the rate at
which energy is transferred
• Power = Energy Transferred
time
Kinetic Energy
• Two basic kinds of energy –
kinetic and potential
• Energy that an object has due
to its motion = kinetic energy
(KE)
• “Kinetos” in Greek means
motion
Factors Affecting
Kinetic Energy
• Depends on both mass and
velocity
• When mass goes up, KE goes
up; when velocity goes up, KE
goes up
• Idea similar to momentum?
Calculating KE
• KE = ½ mv2
• Mass is in Kg
• Velocity is in m/s
• Change in velocity has
greater affect on kinetic
energy than change in mass
Potential Energy
• Stored energy that results from
the position or shape of an
object
• Energy that has the potential to
do work
Gravitational
Potential Energy
• Stored energy that depends on
the height of an object
• The higher an object is the
more GPE it has.
Calculating GPE
• GPE = MGH
• GPE = mass x 9.8m/sec2 x
height
• Mass is in Kg and height is in m
FYI
• Mass * 9.8 m/sec2= Weight (and
weight is a force)
• So we have F * D
Elastic Potential
Energy
• Stored energy associated with
objects that can be stretched or
compressed
• Examples are springs like those
in mattress, a slinky, an elastic
band to put in hair, a rubber
band, a bow to shoot arrows
and many more!
FYI: Calculating EPE
• Technically there is a formula used
to calculate this: PE = ½ kx2 where
“k” is a constant and “x” is the
distance the spring or material is
stretched or compressed from its
original state. You will not be asked
to calculate this type of energy but
you should be aware that it can be
found.
If a car that has a mass of
80kg is rolling down a roller
coaster track, and it has a
velocity of 15m/sec, what is
the Kinetic Energy of the car
at this point in its motion?
• KE = ½ mv2
• KE = ½ 80 kg * (15 m/sec)2
• KE = 9000 Joules
A person with a mass of
58kg is sitting on the top
row of the bleachers 21
meters off the ground at a
football game, calculate his
GPE.
• GPE = mass x 9.8 m/sec2 x
height
• GPE = 58 kg x 9.8 m/sec2 x 21 m
• GPE = 11936.4 Joules
Magic Triangles
KE
.5M
V2
• KE = 0.5 M x V2
• V2 = KE / 0.5M
• 0.5M = KE / V2
Remember
MG =
Weight! (a
force!)
GPE
MG
• GPE = MGH
• MG = GPE / H
• H = GPE / MG
H