Download Modes of Energy

Work, Power, and Modes of
Energy
Work
• Scalar quantity defined as the product
of the force applied to an object and
the displacement of the object.
– W=Fd
Units for work is Joules
J=N·m
If work is scalar, can it be negative?
Yes, if the Force acting on the object is in the
opposite direction of the object’s motion then
work will be negative.
Identify the following as work/nonwork.
A book falls off a table
A teacher
pushing on a
brick wall.
A rocket accelerates
through space.
and free falls to the
ground.
A waiter carries a
tray full of meals
above his head by
one arm straight
across the room at
constant speed.
(Careful!)
Calculating Work
• A 10-N force is applied to push a block across a
friction free surface for a displacement of 5.0 m
to the right.
W=Fd
W=(10N)(5.0m)
W=50J
A force of 50 N acts on the block to move the block a
horizontal distance of 3.0 m. How much work is done
by the applied force?
30
Graphing Work
Force vs Displacement
LxW=
2 m x 25 N =
50 N·m = 50 J
F (N)
• If a 25 N force is applied
to an object for 2
meters, how much work
is done on the object?
– W = Fd =
– (25N)(2m) =
– 50 J
• What is the area under
the graph at right?
20
10
0
0
0.5
1
d (m)
1.5
2
Power
• The rate at which work is done. (Work/Time)
– P=W/t
– P=(Fd)/t
– P=Fv
Metric unit for power is Watts
Watts = J/s
(1000 Watts = 1 kW)
English unit for power
is horsepower (Hp)
1 Hp=746 W
Two physics students, Will N. Andable and Ben Pumpiniron,
are in the weightlifting room. Will lifts the 100-pound barbell
over his head 10 times in one minute; Ben lifts the 100pound barbell over his head 10 times in 10 seconds. Which
student does the most work?
What is Energy (E)?
• The ability to do work in a system.
– Changes in:
• Temperature
• Motion
• Shape
System: object(s) we
focus on in a problem.
Surroundings:
everything else!
Energy is kinda like a substance:
You can measure it. (Joules, calories, BTUs)
It can be stored in a variety of ways.
It can be transferred.
It is universal.
Kinetic Energy (K)
• “moving” energy
– Anything in motion has KE
•
•
•
•
Wind
Rain
Cars
People
• Dependent on m and v
– But….velocity has more of an effect than mass.
Potential Energy (U)
• Energy because of position.
• Several Types:
Gravitational Potential Energy
(UG)
Energy due to Fg.
Comes from mass, height, and g.
The higher off the ground, the
more UG.
No height, No UG.
Elastic Potential Energy (US)
Energy due to stretching or
compression.
Often transferred to other
objects causing them to move.
Chemical Potential Energy
• Energy stored in the chemical bonds of
atoms.
– Food
– Batteries
– Wood
– Fuel
• If it can undergo a chemical change then it has
PEchem
Dissipated Energy
• Energy that produces random movement of
molecules, light, or sound.
– Usually due to friction or deformation of objects.
– Non-reusable form of energy.
Where
does all
that
energy
go?