Work Energy and Power Work is done when

Work, Energy and Power

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. 5 m? Force = Weight = 20 X 9. 8 = 196 N Work = Force X Distance = 196 X 6. 5 = 1274 J

Calculate the work done climbing the ladder.

Example Find the work done bringing a 3000 Kg car from 30 m/s to rest in 10 s. Work = force X distance Force = decelerating force = ma Need to find acceleration and distance u=30 t = 10 v = 0 v=u+at 0=30 +(a)(10) 10 a = -30 a=3 m/s 2 F = 3000 X 3 F = 9000 N v 2 = u 2 + 2 as 02 = 302 + 2(3)(s) 6 S = 900 S = 150 m Work = F X S = 9000 X 150 Work= 1350000 J

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

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

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. 5 m. Calculate their maximum speed.

Power is the rate at which work is done Power = Unit : Watt Example A weight lifter can lift 30 Kg through a height of 60 cm. He does this 100 times a minute. Find his average power. Work = Force X Distance = Weight X distance (30)(9. 8) X 0. 6 = 176. 4 J for one lift 17640 J for 100 lifts Power = 17640 60 = 294 W

Efficiency Percentage Efficiency = Power Output X 100 Power Input Example An engine has an input power of 4500 W and the useful power is 3000 W. Calculate it percentage efficiency. Percentage Efficiency = 3000 X 100 = 66. 67% 4500

Example A crane with a 60 k. W motor lifts a mass of 4000 kg a height of 10 m in 20 s. Calculate the percentage efficiency of the crane. Power output = work = force x distance Time = 4000 x 9. 8 x 10 = 19600 W 20 % Efficiency = 19600 x 100 = 32. 67% 60000