1. Different kinds of energy
Definitions 1
Energy
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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. Different kinds of energy
Definitions 2
Energy
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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. Work = force  distance moved (W = F  s)
Energy
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Work
If a force F is used to move an object for a distance s as shown, we define F s
Work = force  distance moved (W = F  s)
Work is a scalar. It has magnitude only.

4. Properties of work Work 2 Energy
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Properties of work
If the force F is not in the same direction of the object moves, we consider the component of F, s 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. Kinetic Energy Kinetic Energy 1 Energy
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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. F s u v

6. If the object starts from rest (u = 0),
Kinetic Energy 2
Energy
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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. Potential Energy Potential Energy 1 Energy
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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.) h
F = mg
weight = mg

8. If the object starts on the ground,
Potential Energy 2
Energy
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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. Two different principles about energy
Conservation of energy 1
Energy
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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. Different Examples Conservation of energy 2 Energy Elastic collision
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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
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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. 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.
pushing force
fixed wall
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. 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.
direction of motion
holding force
weight

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

16. Conservation of energy 2 Energy
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Conservation of energy 2
Energy
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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. Conservation of energy 2 Energy
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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

18. Conservation of energy 2 Energy
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Conservation of energy 2
Energy
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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
This is the amount of K.E. gained by the object.
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. Conservation of energy 2 Energy
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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

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

21. Conservation of energy 2 Energy
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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

22. Conservation of energy 2 Energy
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Conservation of energy 2
Energy
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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. 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?
friction 1000 N

24. Conservation of energy 2 Energy
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Conservation of energy 2
Energy
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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 = J