1. Chapter 9
Energy

2. 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.

3. 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)

4. x

5. 2. POWER Watt 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 = W
100 W bulb = hp
250 hp engine = 186,450 W

6. Chapter 9 Review Questions

7. A 10 lb weight is lifted 5 ft. A 20 lb weight is lifted 2. 5 ft
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

8. Two cars, A and B, travel as fast as they can to the top of a hill
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

9. Two Types of 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

10. Potential Energy Energy of position or configuration
Examples - Springs, bow, sling shot, chemical energy, and gravitational potential energy
The latter is GPE = mgh

11. 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.

12. Kinetic Energy It is an energy of motion. It is a square law.
Total Work (work done by all forces acting on mass m) = DKE

13. Work to Stop KE
Note

14. 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

15. Chapter 9 Review Question

16. A 20 pound weight is lifted 4 feet
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

17. 4. CONSERVATION OF ENERGY
Galileo's inclines
Demo - Bowling ball pendulum
Demo - Loop the loop
Energy lost due to friction is actually not a loss; it is just a conversion of mechanical energy to heat energy.

18. 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)

19. Perigees
Hyperbola
Parabola
Circle
Ellipse
Ellipse
Apogees
Energy is conserved along
all of these paths.

20. 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

21. No work is required to maintain circular motion at constant speed.
Example is Earth’s orbit around the sun (approximately circular, but actually an ellipse).

22. Einstein’s famous equation allows mass to be destroyed to create energy, and energy to destroyed to create mass.
This is Mass-Energy Equivalence.

23. Chapter 9 Review Question

24. An object of mass 6 kg is traveling at a velocity of 30 m/s
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

25. 5. Machines If no energy losses then: Work Input = Work Output
(F.d)input = (F.d)output
Examples - levers, block and tackle, etc.

26. D F F = D D D

27. 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.)

28. 7. COMPARISON OF KINETIC ENERGY AND MOMENTUM
KE is a scalar and cannot be canceled.

29. This is the impulse applied to the man.
Thief absorbs all the kinetic energy.
This is the impulse applied to the bullet.

30. - Thief does not absorb all the kinetic energy in this latter example.
This is the impulse from previous slide.
This is the impulse applied to the man.
This is the impulse applied to the bullet.

31. 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.

34. Twice the mass at half the speed. Momentum can cancel.

35. 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

36. Punch is the same
but the energy delivered is not.

37. 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