Weight, mass and gravitational field strength

In this lesson

Calculate the weight of an object from its mass and the gravitational field strength, and say which of those three quantities changes when the object is moved somewhere else.

What it covers

• Weight as the FORCE on an object because of gravity - measured in newtons, always acting straight down towards the centre of the Earth
• Mass as the amount of matter in an object - measured in kilograms, and the same everywhere
• Gravitational field strength g, its unit newtons per kilogram, and what it means: the newtons of pull on each kilogram
• Using the value of g PRINTED ON THE PAPER rather than a remembered one, and saying so out loud
• Weight = mass x gravitational field strength: quantity, symbol and unit for all three, in that order
• Converting grams to kilograms BEFORE substituting, written on its own CONVERT line
• The rearrangements m = W / g and g = W / m, each written out as its own line
• Weight and mass are directly PROPORTIONAL - double the mass and the weight doubles - shown as a straight line through the origin on a weight-against-mass graph
• Reading g off that graph as the gradient, with its unit falling out of the axes
• Centre of mass: the single point the whole weight can be treated as acting at, found by symmetry for a simple shape
• Measuring weight with a newtonmeter, which is a calibrated spring balance, and why a shop's weighing scale is responding to a force while displaying a mass
• PHY-123, taught here: the proportional reasoning both ways - what happens to W if m doubles, what happens to W if g halves, and what happens to m if neither

Key words

On the specification