UNIT: ENERGY
I WHAT IS WORK?
A. DEFINITION: Work is a force moving an object over a distance.
(reminder: a FORCE is a push or a pull )
►For work to occur, the object must move in the same direction as the
force is applied
(ACTIVITY: Review and Reinforcement, # 1-10)
B. FORMULA
WORK (W)
= FORCE (N)
x
DISTANCE (m)
the metric unit for work is the joule
one joule (J) = one newton . meter (Nm)
1 Nm = 1 kg . m x
S2
m
or
1 kg . m2
s2
SO…..1J = 1 Nm = 1 kg . m2
s2
D. A WORK EXAMPLE:
A 600 N woman lifts her 100 N suitcase 0.5 m off the floor and walks 25 m.
How much work is her arm doing?
W= ?
work = d x f = 0.5 m x
d = 0.5 m
100 N
= 50 Nm (50 J )
f = 100 N
If the same woman dragged the suitcase, how much work would she do?
W= ?
w = f x d
=
100N x 25 m =
2500 J
d = 25 m
f = 100 N
(ACTIVITY: Calculating Work: Applying the Main Ideas)
II WHAT IS POWER?
A. Definition: Power is the RATE at which work is done.
(Work done per unit of time)
B. Calculating Power:
POWER
=
WORK (J)
TIME (s)
The basic metric unit of power is the WATT (W)
One watt = one joule per second ( J
s
)
A sixty watt (60 W) light bulb is doing work at a rate of 60 joules
per second.)
EX: How much work is done by a lamp with three, 75 W light bulbs,
if the lamp is left on for one hour.
Work = ?
Power = 3 x 75 W
Time= 1 hr x 60 min x 60 s = 3600 s
1 hr 1 min
Work = power x time
= 225 J
s
x 3600 s
= 810, 000 J
III WHAT IS ENERGY?
A. Definition: The ability to do work (that which is necessary to do work.) Remember work is
a force moving an object over a distance—energy is required to create a force.)
B. Units: Joules (J)
IV Kinetic and Potential Energy
1. Kinetic—energy exerted by objects in motion
b. Calculating Kinetic Energy
kinetic energy = ½ x mass x velocity2 =
KE = ½ mv2
Example: What is the kinetic energy of a 6 kg bowling ball rolling at
8.96 m/s ?
m =
v
=
KE =
6 kg
KE = ½ mv2
8.96 m/s
= ½ x 6 kg x (8.96 m/s)2
?
= ½ x 6 kg
=
80.28 m2
S2
2
240.84 kg m (J)
S2
x
2. Potential: Energy of shape or position—energy that is stored.
The potential to move something (the potential to do work!)
a. Gravitational Potential Energy: potential energy that depends on height.
Formula: grav. potential energy = mass x “free fall acceleration” x height
PE = m g h
Example: Wylee Coyote moves a rock to the edge of a cliff, 50 meters above
the Roadrunner. The mass of the rock is 100 kg. What is the gravitational
potential energy of the rock?
m = 100 kg
PE = mgh
g = 9.8 m/s2
PE =
h = 50 m
PE
100 kg x 9.8 m x 50 m
s2
= 49000 kg m2 (joules)
s2
3. Kinetic or Potential? Decide whether each statement describes potential (P) or
Kinetic (K) energy:
_____ a. an airplane circling in preparation for landing.
_____ b. a pile of coal which will be burned.
_____ c. a flag blowing in the wind.
_____ d. a box stored on the top shelf of a closet.
_____ e. an unlit firecracker.
_____ f. a hammer held above a nail.
_____ g. a marble rolling down a ramp.
_____ h. a new car battery.
_____ i. glucose stored in plants as a result of photosynthesis.
_____ j. an ant crawling across a sandwich at a picnic.
_____ k. a tennis racket about to crash down on a ball near the net.
_____ l. an eyelash fluttering.
_____ m. a box of Pop Tarts balanced on the edge of a shelf
_____ n. the Pop Tarts in the box
_____ o. the box of ‘Tarts falling onto your head
V. LAW OF CONSERVATION OF ENERGY
Energy can be neither created nor destroyed—but it can change form.
For example, the potential energy of a boulder on a hillside is converted to the potential energy
of the boulder rolling down the hill.
The chemical energy stored in a chocolate bar is converted to heat energy. Through digestion.
ACTIVITY: video—“Ring of Truth” (Phil Morrison)—2nd half
VI. Energy Conversions IN SYSTEMS
changes from one form of energy to another (e.g., mechanical energy of a car comes from the
chemical energy contained in the fuel.)
A. Kinetic-Potential
Consider the diagram to the right. The circles
represent a ball thrown into the air.
i. Where is the kinetic energy
the highest?
ii. Where does the ball have
maximum potential energy?
iii. Where does the ball have NO
kinetic energy?
iv. What happens to the potential
\energy of the ball as it falls?
v. As the ball gets to maximum
height, what happens to
its kinetic energy?
KEEP IN MIND THAT THE TOTAL ENERGY DOES NOT CHANGE
VII Six Forms of Energy
1. Mechanical—energy of motion
2. Heat (thermal) energy created by the motion of particles colliding with something
else.
3. Chemical—energy released when the bonds between particles are broken. During
a chemical reaction, you will recall that electrons move--new substances
are formed. Even the movement of these tiny particles can create forces. We
know that energy is created/released during chemical reactions because
sometimes we feel it as heat (exothermic, remember?)
4. Electromagnetic—energy that travels in wave form without the need of a medium.
(radio, infrared, visible light, ultraviolet, x-ray, gamma )
5. Nuclear—energy released from the nucleus of an atom. (Remember, the nucleus is
held together by the “strong nuclear force” when this is disrupted, large amounts
of energy are released.)
a. Radioactive decay: When radioactive elements, like uranium, decay,
parts of the nucleus (alpha particles = two protons and two
neutrons together, beta particles = electrons) are thrown out. When
these particles collide with other matter, they exert small forces--we
can say that work is done. (note: gamma radiation, a form of
electromagnetic energy, is also produced.)
b. nuclear fission: the nucleus is split
c. nuclear fusion: nuclei are fused together
6. Electrical—force created by the movement of electrons.
ACTIVITY: Use the six terms (forms of energy) above to describe the pictures that
follow.
VIII MULTI-STEP ENERGY CONVERSIONS
Lab: energy conversions