Download Energy - SFA Physics and Astronomy

Chapter 7
Energy
Energy
Universe is made up of matter and energy
Energy is the mover of matter.
Energy has several forms:
Kinetic, Potential, Electrical,
Chemical, etc.
Work

Work = Force X Distance
W = Fd

Measured in units of Newton-meters or
Joules
Work Questions

How much work is done when a weight
lifter lifts a barbell weighing 1000 N to a
height of 1.5 m?

How much work is done when a weight
lifter pushes on a stationary wall with a
force of 1000 N for 15 seconds?
Power

Power = (work done) / time

Power is measured in units of joules/sec
or Watts
Light Bulbs and Power

How much energy does a 100 W light bulb
use in one hour?
100 W = 100 joules/sec = 360,000 joules/hour
 so in one hour we use,
(360,000 joules/hour)X(1 hour) = 360,000 J
of energy
Mechanical Energy

The ability to do work is called energy and
it has the same units as work (Joules)

Lifting a weight from the floor requires
work (Force * distance). It also gives the
weight energy. (Release the weight and
what happens?)
Kinetic Energy

Energy of Motion

KE = ½ (mass) X (velocity)2

KE = ½ mv2

Question: How much energy does a 1 kg
mass have if it is moving at 10 m/s?
Kinetic Energy

Answer:
m
= 1 kg
v = 10 m/s
= ½ (1 kg) (10 m/s)2
 KE = 50 J
 KE
Kinetic Energy Question

You have a choice of catching a baseball
or a bowling ball, both with the same KE.
Which is safer?
KE = ½Mv2
p1 = Mv
KE = ½mV2
p2 = mV
Solution

m v
 
M V
½Mv2 = ½mV2 , so
But
p1 Mv

p 2 mV
M

m
m
M
M

1
m
2
Work - Energy Theorem

If you want to move something, you are
giving it kinetic energy and you must do
work on it.

The work done is equal to the change in
the kinetic energy
W = DKE
Potential Energy

Stored energy is called potential energy

Examples:
Rubber band
Springs
Bow
Batteries (chemical energy)
Gravitational Potential
Energy

PE = Weight X Height
PE = mgh
Gravitational Potential
Energy
Notes:
Only the vertical height matters (you are
doing work against gravity which only
acts vertically). Independent of path
Only the difference in potential energy
matters. You are free to set PE = 0 at any
point. Arbitrary zero point
Total Mechanical Energy
E = KE + PE
Conservation of Mechanical
Energy

Total mechanical energy cannot be
changed so long as the system has no
dissipative forces (friction, air resistance).

If the system is not losing energy to heat,
then mechanical energy is conserved.
Conservative Forces
Gravity
Springs
Simple Machines

Devices that amplify forces
 If there are no losses, then
Work in = Work out
(FD)input = (FD)output
Simple Machines

Levers
F
d
D
f

Inclined Planes
Simple Machines

Pulleys
f
Conservation of Energy

Throw a ball into the air with an initial
velocity of 10 m/s.
– How high does it get?
How High does it Get?
V=0
KE = 0
PE = mgh
E = mgh
½mv2 = mgh
h = v2/2g = (10 m/s)2/2*10m/s2 = 5 m
V = 10 m/s
KE = ½mv2
PE = 0
E = ½mv2
Efficiency

Efficiency = work done/energy used
Useful energy becomes wasted energy when
it is spent in heat. Heat is the graveyard of
useful energy.
Example Questions - Chapter 7
A 10 lb weight is lifted 5 ft. A 20 lb weight is
lifted 2.5 ft. Which lifting required the most
work?
(a) 10 lb weight
(b) 20 lb weight
(c) same work for each lifting
(d) not enough information is given to work
the problem
An object of mass 6 kg is traveling at a velocity of 30
m/s. How much total work was required to obtain this
velocity starting from a position of rest?
a)
b)
c)
d)
e)
180 Joules
2700 Joules
36 Joules
5 Joules
180 N
W = DK
W = ½mv2 – 0
W = ½ (6 kg) (30 m/s)2
W = 3*900 = 2700 Joules
Two cars, A and B, travel as fast as they can to the top of a
hill. If their masses are equal and they start at the same time,
which one does more work if A gets to the top first?
a) A
b) B
c) they do the same amount of work
A 20 pound weight is lifted 4 feet. The change
in potential energy of the weight in ft.lb is
a)
b)
c)
d)
e)
20
24
16
80
5