Energy

1. Definitions 1 Energy Next Slide Different kinds of energy • Kinetic energy Kinetic energy is motion energy. The faster it moves the more kinetic energy it possesses. • Potential energy Potential energy is position energy. The higher the object is above the ground the more potential energy it possesses.

2. Definitions 2 Energy Next Slide Different kinds of energy • Mechanical energy Sum of kinetic energy and potential energy • Work When we lift an object from the ground up to the height of a table, energy is transferred from us to the load (chemical energy to potential energy). The amount of energy transferred is called work.

3. F F s Work 1 Energy Next Slide Work • If a force F is used to move an object for a distance s as shown, we define Work = force  distance moved (W = F  s) • Work is a scalar. It has magnitude only.

4. F F   s Work 2 Energy Next Slide Properties of work • If the force F is not in the same direction of the object moves, we consider the component of F, W = F cos  s • No displacement implies no work is done. Diagram • Force  direction of motion ( = 90) implies no word is done. Diagram

5. F F v u s Kinetic Energy 1 Energy Next Slide Kinetic Energy • Consider a mass m is pulled by a constant force F on a smooth surface by a distance s as shown. The initial velocity and final velocity are u and v respectively.

6. Kinetic Energy 2 Energy Next Slide • The work done calculated is equal to the amount of energy transferred to the object, i.e. increase in K.E. • If the object starts from rest (u = 0), • Work done by net force = change in K.E. • Hence, we define

7. F = mg weight = mg h F = mg weight = mg Potential Energy 1 Energy Next Slide Potential Energy • Consider we lift a mass m up by a height of h from the ground as shown. The minimum force F required is equal to mg so that the mass is moving upwards with constant velocity (no change in K.E.)

8. Potential Energy 2 Energy Next Slide • The work done calculated is equal to the amount of energy transferred to the object, i.e. increase in P.E. In this case, there is no increase in K.E. (Why?) • If the object starts on the ground, • Hence, we define

9. Conservation of energy 1 Energy Next Slide Two different principles about energy • Conservation of energy Energy can never be created or destroyed. It can only be changed from one form to another. • Conservation of mechanical energy When there is no collision and/or friction, the sum of K.E. and P.E. conserves provided that no work is done by external force.

10. Conservation of energy 2 Energy Next Slide Different Examples • Elastic collision Calculation • An object is pulled on a rough surface Calculation • An object is falling from a height Calculation • Energy changes in a simple pendulum Calculation

11. Power 1 Energy Next Slide Power • Power is defined as the rate at which energy is transferred with respect to time • A car is moving on a rough road Calculation

12. END of Energy

13. fixed wall pushing force Back to Work 2 Energy Click Back to • A person is pushing a fixed wall as shown. No matter how hard he pushes, no work is done. • However, he feels tired and his chemical energy has been used up. All the chemical energy has been changed to internal energy of his body. He feels very hot.

14. holding force direction of motion weight Back to Work 2 Energy Click Back to • A person is now holding a load and moving forward. Again no work is done on the load. It is because the direction of the force is perpendicular to the direction of motion.

15. mass 2 kg mass 2 kg Case 1 mass 2 kg mass 2 kg Case 2 mass 2 kg mass 2 kg Conservation of energy 2 Energy Next Slide • Two objects (mass 2 kg) are moving towards each other as shown with speed 3 m s-1. • After collision,

16. Back to Conservation of energy 2 Energy Click Back to • In both cases, the conservation of momentum holds. • In case 1, there is no loss in total K.E. in the collision, it is an elastic collision. Elastic collision is a collision such that no loss in total K.E. • In case 2, there is loss in total K.E. in the collision, it is not an elastic collision.

17. 10 N 10 N 4 N 4 N rough surface 6 m Conservation of energy 2 Energy Next Slide • An object (mass : 2 kg) is pulled by a force 10 N on a horizontal rough surface with friction 4 N for 6 m. The initial velocity is zero and the final velocity is 6 m s-1. (Why?!)

18. This is the amount of K.E. gained by the object. Back to Conservation of energy 2 Energy Click Back to • Work done by the pulling force = 10 N  6 m = 60 J • This is the amount of chemical energy used by the man. • Work done by the net force = (10 - 4) N  6 m = 36 J • Work done against friction = 4 N  6 m = 24 J • This is the amount of internal energy gained by the surface. It is part of energy input by the man. • Conservation of M.E. does not hold since friction exists.

19. 5 m v Conservation of energy 2 Energy Next Slide • An object (mass 2 kg) are released from rest from a height 5 m. What is its speed just before it hits the ground? Assume the air resistance can be neglected.

20. Back to Conservation of energy 2 Energy Click Back to • There is no friction (air resistance) and external force, so we can apply the conservation of M.E. • By conservation of M.E.

21. 0.2 m ground Conservation of energy 2 Energy Next Slide • An object (mass 2 kg) is released from rest from a height 0.2 m higher than the lowest point in a simple pendulum, what is its speed when it is at the lowest point? Assume the air resistance can be neglected.

22. Back to Conservation of energy 2 Energy Click Back to • There is no friction (air resistance). Moreover, although we have external force (tension), it does no work on the object since it is always perpendicular to the direction of motion of the object, so we can apply the conservation of M.E. • By conservation of M.E.

23. friction 1000 N Power 1 Energy Next Slide • A car (mass 1000 kg) is now moving on a rough horizontal road with a constant velocity 20 m s-1 . The friction acting on the car by the road is 1000 N. What is the force generated by the engine? What is the power of the engine?

24. Back to Conservation of energy 2 Energy Click Back to • By Newton’s second law, the force generated by the engine must be exactly equal to the friction, that means 1000 N. • In 1 s, the car has traveled for 20 m. • In 1 s, the work done against friction = 1000 N  20 m = 20000 J