1. Work, Power, and Machines
Chapter 13

2. Work done when a force acts on an object
AND it moves in the direction of that force
Work = force x distance
W = Fd
work is 0 when an object isn’t moving
measured in joules (J)

3. Practice Problem: Work
A father plays with his daughter by lifting her
repeatedly in the air. How much work does he do with
each lift if he lifts her 2.0m and exerts an average force
of 190 N?
Given: Equation: Solution:
F = 190N W = Fd W = 190N x 2.0m
d= 2.0m W = 380 N ∙ m
W =? W = 380 J
(N ∙ m = J)

4. Power
rate at which work is done (how much is done in a given amount of time)
Power = work
time
P = W t
measured in watts (W)
! don’t confuse W for work with W for watt
!! 1 watt = 1 joule/second

5. Practice Problem: Power
Lifting an alevator 18m takes 100 kj. If doing so
takes 20s, what is the average power of the
elevator during the process?
Given: Equation: Solution:
W = 100kJ 1000J P = W P = 1 x 10 ⁵J
1 kJ t s
t = 20s P = 5 x 10³J/s or
P = ? kw