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Energy
Ch. 5
Energy
I. Energy – the ability to do work (or cause
change)
Most energy can be classified into 2 general
categories:
Energy
1. Potential energy (PE)
– stored energy or
energy of position
PE = mgh
mass (kg)
9.8 m/s2
height (m)
Energy
2. Kinetic Energy (KE) –
energy of motion
KE = ½
mass (kg)
2
mv
velocity (m/s)
Energy
3. The unit for
energy is
Joules (J).
James Prescott Joule
1818-1889
Energy Types
Type
KE
vs
PE
Description
Mechanical Both Energy associated with
motion and position
Energy
of everyday things
Thermal
(Heat)
Energy
KE
Examples
Speeding train,
Bouncing ball,
Sprinting athletes
Energy is the motion of Candles, electric
particles (faster = more
heaters
heat)
Energy Types
Type
KE
vs
PE
Description
Chemical PE
Energy
Stored in chemical
bonds in compounds
(broken bonds – PE
Increases)
Radiant
Energy
Energy of
electromagnetic
waves, including light
KE
Examples
Gasoline,
food, battery
Solar energy
X-rays
Radio waves
Energy Types
Type
KE
vs
PE
Sound
Energy
KE
Electrical
Energy
KE
Description
Examples
Music,
Energy that moves
through substances Pencil hits floor,
etc
(compression waves)
Movement of electrical
charges
Electricity in
wires,
lightning
Energy Types
Type
Nuclear
Energy
KE
vs
PE
Description
PE Energy that is stored+in
the nucleus (holds p &
neutrons together)
Examples
Sun (fusion),
Powerplants
(fission)
Law of Conservation of Energy
A. Energy may change forms but it cannot be
created or destroyed
B. If energy is conserved, then DPE = DKE (D is
“change in”)
C. Conservation of Mechanical Energy:
(KE + PE)beginning = (KE + PE)end
D. Energy flowchart shows how energy changes
form.
Law of Conservation of Energy
a. The life of Stretchy the Rubber Band:
Stretchy is pulled – KE  PE
Stretchy is released – PE  KE
Stretchy is broken –
KE  heat & sound
Energy Problems
Examples:
What is the potential energy at points A through E?
Energy Problems
To demonstrate a cheerleading stunt, Miss
Rokosik has to lift Miss Kalousek over her
head (to a height of 2 m). Miss Kalousek
has a mass of 50 kg. What is the potential
energy of Miss Kalousek?
Energy Problems
What is Miss Kalousek’s kinetic energy while
she is held above the ground?
Energy Problems
Miss Rokosik gets tired and drops Miss
Kalousek from the 2 m height. What is her
kinetic energy just before she hits the
ground? How do you know?