What is work?

• Work is the transfer of energy to a body by the application of a force that causes the body to move in the direction of the force.

• Work is done only when a force causes an object to move in the direction of the force. This is different from the everyday meaning of work.

• Work Equation

work = force ´ distance

W = F ´ d

• Work is measured in joules.
• Because work is calculated as force times distance, it is measured in units of newtons times meters, N•m.
• These units are also called joules (J). In terms of SI base units, a joule is equivalent to 1 kg•m2/s2.

• Definition of joules

1 J = 1 N•m = 1 kg•m2/s2

EXAMPLE

Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N?

• List the given and unknown values.

Given: force, F = 190 N

distance, d = 2.0 m

Unknown: work, W = ? J

Write the equation for work.

work = force ´ distance

W = F ´ d

Insert the known values into the equation, and solve.

W = 190 N ´ 2.0 m = 380 N•m = 380 J

Power

• Power is a quantity that measures the rate at which work is done or energy is transformed.

• Power Equation

Power = work/time (p=w/t)

• Power is measured in watts.

A watt (W) is equal to a joule per second (1 J/s).

EXAMPLE

(Power) It takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process?

1. List the given and unknown values.

Given: work, W = 100 kJ = 1 ´ 10 5 J

time, t = 20 s

The distance of 18 m will not be needed to calculate power.

Unknown: power, P = ? W

2. Write the equation for power.

work = force ´ distance

W = F ´ d

Insert the known values into the equation, and solve.

P= 1 ´ 10 5 J/20 sec = 5 ´ 10 3 J/s = 5 ´ 10 3 W = 5 kW