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)
x
F
xFW x
xF
yF
Video ClipVideo Clip
Manpowered MachinesManpowered Machines
2. POWER
Power = Work/time or P = W/t
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
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 EnergyPotential EnergyKinetic Energy
Potential Energy
Energy of position or configuration
Demo - Dart gunDemo - 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 ClipsVideo Clips
InclineIncline
ScrewScrew
Kinetic Energy
It is an energy of motion.
KE mv 12
2
It is a square law. Total Work (work done by all forces acting
on mass m) = KE
221 )2( ivm
Work to Stop KE
)(4 221
imv
xFmvmv xif 2212
21
xFmv xi 221
0
221 4 ivm xxF
Note
Work-Energy Theorem
The net work done on an object is equal to the change in the kinetic energy of the object.
Net Work = KE
4. CONSERVATION OF ENERGY
Demo - Bowling ball pendulumDemo - Bowling ball pendulum
Demo - Loop the loopDemo - Loop the loop
Video - Pole VaultingVideo - Pole Vaulting
Galileo's inclines
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)
Ellipse
Ellipse
ParabolaH
yperbola
Energy is conserved alongall of these paths.
Perigees
Apogees
Circle
Video ClipsVideo Clips
Driving NailsDriving Nails
Water WheelWater Wheel
Roller CoasterRoller 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.
2mcE
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 - Block and tackle
Demo - Hydraulic liftDemo - Hydraulic lift
FD = FD
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.
ip
0fp
I
Thief absorbs all the kinetic energy.
if ppp
This is the impulse applied to the bullet.
This is the impulse applied to the man.
ip
Thief does not absorb all the kinetic energy.
fp ip
-
I
if ppp
This is the impulse applied to the bullet.
This is the impulse applied to the man.This is the impulse from previous slide.
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.
0)2()2( vmvm
Twice the mass at half the speed. Momentum can cancel.
2mv21 )2(
21 vmKE
22 )2(
21 vmKE
Kinetic energy is not a vector and cannot cancel out. The kinetic energy of the big slow person is
The kinetic energy of the small fast person is
22mv
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
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
(c) same work for each lifting
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
(c) they do the same amount of work
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
(b) 2700 Joules
A 20 pound weight is lifted 4 feet. The change in potential energy of the weight inft.lb is(a) 20
(b) 24
(c) 16
(d) 80
(e) 5
(d) 80