Work
and
Energy
ENERGY
The ability to
do work
WORK
A change in energy
Work Energy
Circular
Definition!
Radiant
Thermal
Electrical
Nuclear
Chemical
Sound
Mechanical
Magnetic
At home – check out
http://www.youtube.com/watch?v=3ff_
AXVlo9U for the Singing Tesla Coil!
Potential energy + Kinetic energy
=
Mechanical energy
Potential energy + Kinetic energy = Mechanical energy
Example of
energy changes
in a swing or
pendulum.
2 Kinds of
Mechanical Energy
• Kinetic energy (KE)
• Potential Energy (PE)
–Gravitational PE (PEg)
–Elastic PE (PEelastic)
Kinetic Energy
Energy due to
motion
KE = ½
2
mv
Potential Energy
Energy due to
position
Bowling Ball –
Conservation of Energy
Conserving Energy
Gravitational PE
PEg = mgh
A 25 kg boy runs with a speed of 6.5
m/s. How much kinetic energy does
the boy have?
530 Joules
How much potential energy does a
0.50 kg apple have if it hangs from a
tree 13m above the ground?
64 Joules
A 220 gram fly has a kinetic energy
of 55 Joules. How fast is the fly
flying?
22 m/s
ENERGY IS
CONSERVED!!
WORK
W = FDs
(Work = Force x displacement)
Units:
Joule (J) = N· m
Work is done only
when components of
a force are parallel
to a displacement.
Force helps
motion
Force opposes
motion
Force 90°
to motion
Object not in
motion
Positive
Work
Negative
Work
No work
No work
A student drags his book
bag 15 meters across the
classroom by pulling on it
with a force of 5.0 Newtons.
How much work does the
student do on the bag?
75 Joules
Larry carries his 35N
physics book across the
room 25m away. How
much work does he do on
the book?
0 Joules
Work-Kinetic Energy
Theorem
Wnet = DKE
POWER
The rate at which
work is done
W
P= Dt
Or…
DKE
P= Dt
Or…
DPE
P= Dt
Or…
P = Fv
= Force * velocity
The SI unit of power
is the Watt,
named in honor of
James Watt.
One Watt, W, of power
is the power achieved
when 1.0 J of work is done or
1.0 J of energy is transferred
in a time of 1.0 s.
Unit of Power
Watt = 1 J/s
Horsepower (hp) = 746 W
How much power is generated
by a motor that can lift a 5300
kg crate of bricks with a
velocity of 15 m/s?
780,000 W
How much power is
generated by a car engine
that does 5500 Joules of
work in 35 seconds?
160 W
How long will it take a
36 Watt motor to pull a
box across the floor 25 m
with a force of 16 N?
11 s
Homework:
Chapter 8, Hewitt:
Read 8.1-8.6
Do RQ 1-13;
P&C 1-3,5-7;
T&E 3-8;
T&S 1-4