Weight
Mass is a measure of how much a body resist a change to its motion. In everyday words, this means that it is harder to push a body with larger mass.
A completely different observation is that two bodies can attract each other. This is called gravitational attraction. It is most obvious between the earth and an body on earth, like yourself. We know that it also happens between the sun and the earth. In fact, it happens between any two bodies big or small, such as between two books. We also know that if the bodies have larger masses, they attract with a larger force of gravity.
What has resisting changes in motion got to do with gravitational attraction? The answer is, we don't know. All we know is that larger mass always means larger attraction.
On this page, we are interested in the attraction from earth on a body on earth. This attraction is called the weight of the body. A body with a larger mass also happens to have a larger weight. From measurements of weights and masses of many bodies, it is found that they are related by:
where W is the weight of a body in N, m the mass in kg, and g the acceleration of free fall 9.81 m/s2
Imagine holding a book in your hand. You can feel the weight. If you move it up, down, left or right, you can still feel the weight. Somehow, the attraction from the Earth can reach out to the space all around you and attract the stone. The region of space, where a gravitational force acts, is called a gravitational field. This field is not just around you. It surround the whole Earth and extends into outer space.
So we can think of weight as the effect of the gravitational field on the mass on a body.
Copyright 2011 by Kai Hock. All rights reserved.
Last updated: 7 February 2011