Transcript Chapter 5

Chapter 7
Energy
Introduction
Universe is made up of matter
and energy.
Energy is the mover of matter.
It has several forms. To
understand this concept we
will begin with a closely related
physical concept.
1. WORK
Now instead of a force for how long in
time we consider a force for how long in
distance.
Work = Force x distance or W = F.d
Units - Joules (J) or ft.lb
BTU = 778 ft.lb (energy of one wooden
kitchen match)
Pushing on a wall and wall doesn’t move
(no work done on the wall)


F
Fy

Fx
x
W  Fx x
Video Clip
Manpowered Machines
2. POWER
Power = Work/time or P = W/t
Watt
Units - J/s = W
550 ft.lb/s = 1 hp
1 hp = 746 J/s = 746 W
1 BTU/hr = 0.293 W
100 W bulb = 0.1341 hp
250 hp engine = 186,450 W
Chapter 7 Review
Questions
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
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 the most
work if A gets to the top first?
(a) A
(b) B
(c) they do the same amount of work
3. MECHANICAL ENERGY
When work is done on an object, the
object generally has acquired the ability
to do work.
This is called energy and it has the same
units as work.
Two Types of Mechanical Energy
Potential Energy
Kinetic Energy
Potential Energy
Energy of position or configuration
 Demo - Dart gun
Other examples - Springs, bow, sling
shot, chemical energy, and gravitational
potential energy
The latter is GPE = mgh
The potential energy of an object
depends on a reference position.
It represents the work done against
gravity to put the mass m in its position
h above some reference position.
It is an energy of position.
Video Clips
Incline
Screw
Kinetic Energy
It is an energy of motion.
KE  mv
1
2
2

It is a square law.

Total Work (work done by all forces
acting on mass m) = DKE
Work to Stop KE
1
2
0
mv  mv
2
f
1
2
1
2
mv
2
i
2
i
 Fx x
 Fx x
Note
1
2
m( 2vi )  m4v
2
1
2
2
i
x
2
1
4
(
mv
 2 i ) Fx
Work-Energy Theorem
The net work done on an object is equal to
the change in the kinetic energy of the
object.
Net Work = DKE
Chapter 7 Review Question
A 20 pound weight is lifted 4 feet. The
change in potential energy of the
weight in ft.lb is
(a) 20
(b) 24
(c) 16
(d) 80
(e) 5
4. CONSERVATION OF ENERGY
Galileo's inclines
 Demo - Bowling ball pendulum
 Demo - Loop the loop
 Video - Pole Vaulting
Energy lost due to friction is actually
not a loss; it is just a conversion.
Energy cannot be created or destroyed.
It may be transformed from one form
into another, but the total amount of
energy never changes.
Energy Conservation in Satellite Motion
(Next slide)
Perigees
Circle
Ellipse
Ellipse
Apogees
Energy is conserved along
all of these paths.
Video Clips
Driving Nails
Water Wheel
Roller Coaster
Condition for Conservation of
Mechanical Energy
No work can be done on the object by a
nonconservative force.
A nonconservative force is a force that
converts mechanical energy into another
form.
Example: Friction
No work is required to maintain circular
motion at constant speed.
E  mc
2
Chapter 7 Review Question
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) 180 Joules
(b) 2700 Joules
(c) 36 Joules
(d) 5 Joules
(e) 180 N
5. Machines
If no losses then
work input = work output
(F.d)input = (F.d)output
Examples - levers, block and tackle, etc.
Demo - Block and tackle
Demo - Hydraulic lift
F
D
D F
=
D
D
6. EFFICIENCY
Efficiency = work done/energy used
Useful energy becomes wasted energy
with inefficiency.
Heat is the graveyard of useful energy.
EER = energy efficiency ratio
It is the output capacity(BTU/hr)/input
energy(Watts)
(Output capacity represents energy moved.)
7. COMPARISON OF KINETIC
ENERGY AND MOMENTUM
KE is a scalar and cannot be canceled.

pi

pf  0
 

Dp  p f  pi

I
This
Thief
is the
absorbs
impulse
allapplied
the kinetic
to the
energy.
man.
This is the impulse applied to the bullet.

pi

pf
 

Dp  p f  pi

- pi

I
ThiefThis
does
allfrom
the kinetic
energy
isisnot
the
impulse
previous
slide.
This
theabsorb
impulse
applied
to the
man.
in this latter example.
This is the impulse applied to the bullet.
Rubber bullets versus lead bullets
Slow and fast football players with
different masses.
Consider head-on with one having twice
the mass but half the speed of the
other.
Twice the mass at half the speed.
Momentum can cancel.
( 2 m )v  m( 2v )  0
 Kinetic energy is not a vector and cannot cancel
out.
 The kinetic energy of the big slow person is
2
2
1
KE1  ( 2 m )v  mv
2
 The kinetic energy of the small fast person is
2
2
1
KE2  m( 2v )  2 mv
2
Punch is the same
but the energy delivered is not.
8. SOURCES OF ENERGY
Except for nuclear and geothermal power,
the source of practically all our energy is
the sun.
Nuclear power
Geothermal power
Solar power
Wind power
9. ENERGY FOR LIFE
Reading assignment