1. Work, Energy and Power
Chapter 11

2. What is Work?
Suppose a force F moves a body through a distance s in the direction of the force.
The Work done (W) is equal to the force multiplied by the distance.
i.e Work = Force × Distance
W = F s

3. What is the SI Unit of Work?
The SI unit of work is the joule (J).
Work is a Scalar Quantity.

4. Define the joule.
One joule is the amount of work done when a force of 1 newton acts for a distance of 1 metre in the direction of the force.
i.e. 1 J = 1 N m
1 joule of Work is done.

5. A person pushes a car a distance of 20 m with a force of 600 N
A person pushes a car a distance of 20 m with a force of 600 N. Calculate the work done by the person.
Work done = W = F s = (600)(20) = J

6. A man carries a 50 kg bag up steps at a steady speed
A man carries a 50 kg bag up steps at a steady speed. The vertical height through which he rises is 20 m. He exerts an upward force on the bag equal to its weight. Find the work done by the man on the bag.
Force F exerted by man
= Weight of bag
= (50)(9.8) = N
Distance s travelled in direction of
force
= vertical distance through
which bag moves
= 20 m
Work done = F s
= (490)(20)
= J

7. Considerable work must be done to raise a lift and the people inside it upwards against the force of gravity.

8. What is Energy? Energy is the ability to do work.
The amount of energy something has is the amount of work it can do.

9. What is the SI Unit of Energy?
The SI Unit of energy is the joule (J).

10. What is Kinetic Energy?
Kinetic Energy is the energy a body has due to its motion.
If a body of mass m kg moves with a speed v m s-1, its Kinetic Energy Ek is given by:

11. A dog of mass 6 kg is moving with a velocity of 10 m s-1
A dog of mass 6 kg is moving with a velocity of 10 m s-1. Calculate its kinetic energy.
Kinetic Energy = Ek = ½ m v2
= ½ (6)(10) = J

12. What is Potential Energy?
Potential Energy is the energy a body has due to its position in a force field.
If a body of mass m kg is raised through a height h metres, the increase in its Gravitational Potential Energy, Ep is given by:
Ep = m g h

13. As the water falls vertically it loses Gravitational Potential Energy.
It gains an equal amount of Kinetic Energy.

14. Electromagnetic Energy
Electromagnetic Energy is energy transmitted from one place to another in the form of electromagnetic waves.
Electromagnetic waves travel at the Speed of Light (3108 m s-1 in a vacuum).
The diagram shows the full range of electromagnetic waves in order of increasing wavelength. This is called the electromagnetic spectrum.
Electromagnetic waves are also known as electromagnetic radiation.

15. Mobile phones transmit signals in the form of microwaves, which are a form of electromagnetic energy.

16. State The Principle of Conservation of Energy.
The Principle of Conservation of Energy states that energy cannot be created or destroyed but can only be converted from one form to another.

17. The fire converts chemical energy to heat and light energy.

18. The bow and string have strain potential energy.
When released this becomes the kinetic energy of the arrow.

19. When two balls collide in snooker almost no kinetic energy is lost to other forms.

20. Kinetic Energy of the wind is converted into Electrical Energy.
Wind energy is a renewable source of energy.

21. What is Power? Power is the rate at which work is done. Or
Power is the rate at which energy is transferred.

22. What is the SI Unit of Power?
The SI Unit of power is the watt (W).
1 watt is 1 joule per second

23. Percentage Efficiency = Useful output Power × 100
Power Input
Percentage Efficiency

24. This Trent class RNLI lifeboat has two independent engines and fuel systems. The maximum power output of each engine is watts.

25. A typical domestic incandescent light bulb has an efficiency of 2. 5%
A typical domestic incandescent light bulb has an efficiency of 2.5%. This means that only 2.5% of the electrical power input is given out as light.
Bulb Type
Typical Efficiency
Incandescent Domestic bulb
2.5%
Incandescent Halogen Car headlamp 4%
Compact fluorescent bulb
11%
White LED
16%