Work
Work is done when energy is converted from one form to another.
In pushing a rock, chemical energy in the muscle is converted to kinetic energy of the rock
The work done can be calculated from the formula
W = F x d
where d is the distance moved in the line of the force.
Work is measured in Joules, J
Definition of the Joule
The Joule is the work done when a 1 N force moves through a distance of 1m.
Calculate the work done on an object of mass 5 kg when it falls from a vertical height of 52 m.
F = 5 kg x 10 m/s = 50N
d = 52 m
W = 50 x 52
W = 2600 J
The Efficiency of an energy conversion is calculated using the formula
efficiency = [work output / work input] x 100
If the useful work output from a system is 80 J and the total work input is 100 J
efficiency = (80 / 100) x 100 = 80 %
Power
Power is the rate of doing work
It is calculated using equation P = W / t
P - power (Watts), W - work (Joules), t - time (seconds)
If 15 J of energy is converted to other forms in 3 s, the power dissipated is 15 / 3 = 5 W