Efficiency, Power Notes

Efficiency Work in is the amount of work that is put into the machine by the energy source. This is often a person or a fuel. Work out is the amount of work that is done on some object by the machine. Efficiency tells us how much of the work which is put into a machine is actually applied to the object by the machine.

Efficiency With no friction, efficiency = 100% meaning that all the work put in comes out on the object. With friction, efficiency< 100% meaning that some of the work in is lost to friction. Efficiency = work out / work in

Efficiency For example, with the above inclined plane ideal mechanical advantage = din/dout =5m/2m=2.5 This means that ideally we should get out 2.5 times more force than we put in.

Efficiency Because of friction, the actual mechanical advantage will be less than if friction were absent, and has to be determined experimentally using the input and output forces. Using the early example of the ramp, the actual mechanical advantage = Fout /Fin = 10N/(>4N) = <2.5 This means that in actuality we will get out less than 2.5 times more force than we put in

Efficiency We can use the two mechanical advantages to find efficiency also. Efficiency =(actual mech. advan.)/(ideal mech. advan.) Efficiency can also be defined by the ratio of Energy in to Energy out:

Power When work is put into an object, energy is transferred from one system to another. The rate that this happens is described as Power P = W / time The units for power are Joules/second, which are known as Watts (W) (don’t get confused between W=watts and W=work—one is a unit and the other a symbol)

Power Another way to define power is force multiplied by velocity P = (F)(v) The units work out the same: (Newton) x (meters/seconds) is the same as (Newton x meter) / (s)