2. EnergyDefinitions 1 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. Next Slide

3. EnergyDefinitions 2 Different kinds of energy Mechanical energy Sum of kinetic energy and potential energy 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. Work Next Slide

4. EnergyWork 1 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) Next Slide F F s Work is a scalar. It has magnitude only.

5. EnergyWork 2 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 Next Slide No displacement implies no work is done. s FF  Force  direction of motion (  = 90  ) implies no word is done. Diagram

6. EnergyKinetic Energy 1 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. Next Slide F F s u v

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

8. EnergyPotential Energy 1 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.) Next Slide h F = mg weight = mg

9. EnergyPotential Energy 2 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?) Next Slide If the object starts on the ground, Hence, we define

10. EnergyConservation of energy 1 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. Next Slide

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

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

13. END of Energy

14. EnergyWork 2 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. pushing force fixed wall Back to Click Back to

15. Energy Click Back to Work 2 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. Back to direction of motion holding force weight

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

17. Energy Click Back to Conservation of energy 2 In both cases, the conservation of momentum holds. Back to 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.

18. Energy Next Slide Conservation of energy 2 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?!) 10 N 4 N 6 m rough surface

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

20. Energy Next Slide Conservation of energy 2 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. v 5 m

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

22. Energy Next Slide Conservation of energy 2 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. ground 0.2 m

23. Energy Click Back to Conservation of energy 2 By conservation of M.E. 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.

24. Energy Next Slide Power 1 friction 1000 N 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?

25. Energy Click Back to Conservation of energy 2 In 1 s, the car has traveled for 20 m. 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 work done against friction = 1000 N  20 m = 20000 J