Transcript 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)