Concept Summary Batesville High School Physics

1. Energy Concept Summary Batesville High School Physics

2. 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: • 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. Kinetic Energy • In symbols: 1 KE = mv2 2

5. 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. 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. • 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. Work • If the force on the object is in the direction the object moves, the work done is: W = Fx F x

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

10. 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 • The work done on an object by the net force equals the object’s change in kinetic energy. Wnet = DKE

12. 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. Potential Energy • Potential energy is energy that an object (system) has due to its position or arrangement.

14. 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 • 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. 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 • 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. Mechanical Energy • Mechanical Energy = PE + KE

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

20. 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 • Power is a scalar quantity.

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

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

24. 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 • 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. The End