1. Ch 8 Energy Notes Concept Summary Batesville High School Physics

2. Ch 8 Energy Notes
Kinetic Energy
If an object is moving, it has energy. (Be careful, the converse of this statement is not always true!)
This energy is called kinetic energy - the energy of motion.

3. Kinetic Energy An object’s kinetic energy depends on:
Ch 8 Energy Notes
Kinetic Energy
An object’s kinetic energy depends on:
the object’s mass.
Kinetic energy is directly proportional to mass.
the object’s speed.
Kinetic energy is directly proportional to the square of the object’s speed.

4. Ch 8 Energy Notes
Kinetic Energy
In symbols: 1
KE =
mv2 2

5. Kinetic Energy Kinetic energy is a scalar quantity.
Ch 8 Energy Notes
Kinetic Energy
Kinetic energy is a scalar quantity.
Common units of kinetic energy: Joules
An object with mass of 1 kg, moving at 1 m/s, has a kinetic energy of 0.5 Joule.

6. Ch 8 Energy Notes
Work
When the kinetic energy of an object changes, work has been done on the object.
Units of work: Joules
Work is a scalar quantity.

7. Work Work depends on: The amount of force applied to the object.
Ch 8 Energy Notes
Work
Work depends on:
The amount of force applied to the object.
The distance that the object moves while the force is applied.
The direction of the force with respect to the direction the object moves.

8. Ch 8 Energy Notes
Work
If the force on the object is in the direction the object moves, the work done is:
W = Fx F x

9. Ch 8 Energy Notes
Work
If the direction of the force is opposite the direction the object moves, work is:
W = -Fx F x

10. Ch 8 Energy Notes
Force is NOT Work
If the force is perpendicular to the direction the object moves, the work done is 0.
If the object doesn’t move, the work done is 0. F
W = 0 x

11. Work and Kinetic Energy
Ch 8 Energy Notes
Work and Kinetic Energy
The work done on an object by the net force equals the object’s change in kinetic energy.
Wnet = DKE

12. Ch 8 Energy Notes
Potential Energy
Sometimes work is not converted directly into kinetic energy. Instead it is “stored”, or “hidden”.
Potential energy is stored energy or stored work.

13. Ch 8 Energy Notes
Potential Energy
Potential energy is energy that an object (system) has due to its position or arrangement.

14. Calculating Potential Energy
Ch 8 Energy Notes
Calculating Potential Energy
To calculate the potential energy of a particular arrangement:
Pick a position or arrangement that you want to call the “potential energy = 0” situation.

15. Calculating Potential Energy
Ch 8 Energy Notes
Calculating Potential Energy
The potential energy of any other position or arrangement equals the negative of the work that the conservative force does in changing from the potential energy = 0 situation to that one.
PE = - WorkF

16. Ch 8 Energy Notes
Conservative Forces
Energy or work is stored when a force does work “against” a force such as the gravitational force or a Hooke’s Law (spring) force.
Forces that store or hide energy are called conservative forces.

17. Gravitational PE GPE = mgh
Ch 8 Energy Notes
Gravitational PE
The gravitational potential energy of an object at height h equals the negative of the work that gravity does when the object is lifted from the PE = 0 position.
GPE = mgh

18. Ch 8 Energy Notes
Mechanical Energy
Mechanical Energy = PE + KE

19. Conservation of Energy
Ch 8 Energy Notes
Conservation of Energy
If no external forces act on a system, the total energy of the system will remain constant.

20. Power DWork Power = time W P t Power is the rate work is done.
Ch 8 Energy Notes
Power
Power is the rate work is done.
DWork
Power =
time W P t

21. Power Units of power: 1 Joule/sec = 1 Watt 1000 Watts = 1 kilowatt
Ch 8 Energy Notes
Power
Units of power: 1 Joule/sec = 1 Watt
1000 Watts = 1 kilowatt
Power is a scalar quantity.

22. (Simple) Machines A machine is a mechanical device used to do work.
Ch 8 Energy Notes
(Simple) Machines
A machine is a mechanical device used to do work.
Examples of simple machines:
Inclined plane
Lever
pulley

23. Ch 8 Energy Notes
(Simple) Machines
A machine can never output more work (energy) than is put into it.
At best,
Workout = Workin
Machine
Workout
Workin

24. Ch 8 Energy Notes
Mechanical Advantage
Machines can’t multiply work or energy, but they can multiply force. Mechanical advantage measures how much a machine multiplies force.
Force machine exerts
MA =
Force you exert

25. Efficiency Useful work done Efficiency = x 100% Energy input
Ch 8 Energy Notes
Efficiency
The efficiency of a machine tells how much of the energy (work) that goes into the machine actually does useful work.
It is usually expressed as a percent.
Useful work done
Efficiency =
x 100%
Energy input

26. Ch 8 Energy Notes
The End