Work & Machines

Work Transfer of energy that occurs when a force makes something move No movement = no work!! 2 conditions: object must move movement must be in same direction as applied force

No work being done because force applied to books and backpack is upward but movement is forward.

Work & Energy When work is done, a transfer of energy occurs Energy is the ability to cause change so…. energy is the ability to do work!! Transfer of chemical energy in muscles to the box = from object doing work to object on which the work is done

calculating work Work depends on force exerted and the distance over which the force is applied Formula: W = Fd Ex: you push a refrigerator with a force of 100N over a distance of 5m, how much work do you do? W = F = D = Measured in Joules

A pitcher exerts a force on the ball only when his hand is touching it, so only work is done on the ball for a short moment

Power The rate at which work is done Can do same amount of work but have different amount of power Girls carrying same amount over same distance but one took 30 seconds and the other 40 seconds

calculating power Formula: P = W/t Power (in watts) = work (in joules) time (in seconds) Ex: You do 900J of work pushing a sofa, how much power did you use if you took 5s? P = W = t = Power is measured in watts (W)

power & energy Power is also the rate at which energy is transferred P = E/t Formula: P = energy transferred (in joules) time (in seconds) Ex: a cell phone uses 110W in 30minutes Measured in joules/second (j/s)