1. Work, Energy and Power

2. Work
Work is done when a force moves a body Work = force X distance Unit : The Joule Example : How much work is it to lift a 20 kg mass vertically 6.5m? Force = Weight = 20 X 9.8 = 196 N Work = Force X Distance = 196 X 6.5 = 1274 J

4. Calculate the work done climbing the ladder.

5. Example
Find the work done bringing a 3000Kg car from 30m/s to rest in 10s.
Work = force X distance
Force = decelerating force = ma
Need to find acceleration and distance
v2 = u2 + 2as
02 = (3)(s)
6S = 900
S = 150m
Work = F X S
= 9000 X 150
Work= J
u=30 t = 10 v = 0
v=u+at
0=30 +(a)(10)
10a = -30
a=3m/s2
F = 3000 X 3
F = 9000N

6. Energy
Energy is the ability to do work. Unit : Joule Kinetic energy is energy due to movement. Kinetic Energy = ½ mass x (speed)2 = ½ mv2

7. Potential energy is energy due to
position or condition
Potential Energy = mass X g X height

8. The Principle of conservation of energy.
Energy cannot be created or destroyed but is converted form one form to another.
Example: The highest point reached by a child on a swing is 1.5m. Calculate their maximum speed.

10. Power
Power is the rate at which work is done Power = Unit : Watt Example A weight lifter can lift 30Kg through a height of 60cm. He does this 100 times a minute. Find his average power.
Work = Force X Distance = Weight X distance
(30)(9.8) X 0.6 = J for one lift
17640 J for 100 lifts
Power = = W 60

11. Efficiency Percentage Efficiency = Power Output X 100 Power Input
Example
An engine has an input power of 4500W and the useful power is 3000W. Calculate it percentage efficiency.
Percentage Efficiency = 3000 X 100 = 66.67%
4500

12. Example A crane with a 60kW motor lifts a mass of 4000kg a height of 10m in 20s. Calculate the percentage efficiency of the crane. Power output = work = force x distance Time Time = 4000x9.8x10 = W 20 % Efficiency = x 100 = 32.67% 60000