Watts a Joule


Calculate the work and power done to lift a 2 Liter bottle of water.


2 Liter Bottle full of water, cap on.
Meter stick or 1 meter tape.
Stopwatch or clock with sweep second hand

To Do and Notice

Lift the bottle one meter

Lift the bottle 1 meter in 1 second

What's Going On?

When you lift the bottle against gravity you are exerting a force on the bottle over the distance of one meter.

Physicists say you are doing work on the bottle.

Work done in a time interval is power.

Math Root

What is the mass, m, of a 2L bottle full of water? ( m is about 2 kg)

What is the gravitational force, F, on the bottle of water

F = mg = 2 * 10 = 20 N or 20 newtons.

where g is the acceleration of gravity 9.8 m/s^2 which we round off to 10 m/s^2

What is the work done, W, to lift the bottle a distance d = 1 meter?

W = F * d = 20 N * 1 m = 20 J or 20 joules

What is the power, P, to lift the bottle 1 meter in time t = 1 second?

P = W/t = 20J/1s = 20 W or 20 watts


Scientific Explorations by Paul Doherty

© 2014

30 May 2014