Work, Power, and Efficiency

Work Work is a measure of a transfer of energy. In order for work to be done, something MUST move in the SAME direction as a force is applied. The unit for work is the Joule or the Newton- Meter Work=Force x Distance w=F * d

Examples Is work being done while a student pushes against the wall? Think, is there a force? (yes) Is there a change in distance? (no) Is work being done while a student carries books from my room to another room on the same floor? Think, what force are you fighting against? Is the direction you’re moving the same as the direction the force is applied?

Power Power is a rate of work. Power= Work/Time P=w/t The unit for power is the Watt OR the Joule per Second (J/s) OR the Newton-Meter per second (Nm/s)

Efficiency Efficiency is a term applied to machines. Machines make work easier for us, but they do not create work or energy. This means that the efficiency of a machine can never be 100% or larger. Machines can change the amount of force or distance you must move something. They can also change the direction.

Efficiency Efficiency=(workout/workin)*100 Divide the work that the machine does by the work you put into the machine then multiply by 100 in order to get a percentage. E=(wout/win)*100 Remember since w=F*d, that: E=((Fout*dout)/(Fin*din)) Depending on the information given, you can use either formula to get info about a machine.

Example Problems How much work is required to push an object with 90N of force a distance of 5 meters? (w=F*d w=90*5=540J) How much force is needed to push an object 15 meters if you only exert 45J of energy? (w=F*d 45=F*15 F=45/15=3N) How much power is used by someone who exerts 900J of work in 45s? (P=w/t P=900/45 P=20W)