Explanation of Mass and Weight.

This may amuse you but every time I see reference to weight I think about mass and wonder if the writer really means what he says.

So, to avoid confusion and loads of emails coming from all and sundry, here goes with my explanation.

Weight:

This is the easy one. It is the force you need when you try to lift the object, to move it about or to keep it in one place.

On the earth's surface, gravity is pulling everything down and to keep an object even in a fixed place you need to apply a steady upwards force all the time.

If you let go of an object it will free-fall and have no weight at all while it is falling. A dropped object is in fact 'in-orbit' and weightless. If you throw a stone horizontally the weight increases temporarily while you are accelerating it and once you let go the weight falls to zero while it is curving down towards the ground in a curve called an ellipse. If the earth was a point mass, at the centre of the earth, then the object would continue to fall down towards the centre, gradually increasing in speed and then just miss the centre and whip around real fast before coming back up again to reappear ready for you to catch. The path traced out is a very long elongated ellipse, with one of the focal points at the earth's centre. The same applies for all satellite earth orbits except that most of the ones we are familiar with involve ellipses that are broad enough and wide enough to miss the earth completely plus 100 miles or so to keep clear of atmospheric drag. Ref: Johannes Kepler: German astronomer who formulated laws describing the elliptical orbits of the planets about the sun, born in Weil der Stadt, 1571.

You may be told that the path followed by a thrown ball is a parabola. This would be true if the direction of gravity was the same,. i.e parallel, across the range of the flight. Over short distances this is almost true. Over longer distances you need to realise that gravity is vertical relative to the horizontal ground and that the earth's surface is curved in an approximate sphere.

In the metric system, force is measured in newtons, hence weight is also measured in newtons.

On the surface of the earth, a mass of 1 kilogram weighs about 9.8 newtons or 1 gram mass weighs about 0.00098 newtons. The weight is less at the equator as the earth spins around and tries to throw things off a bit so quite what your brick weighs depends on where you are. Try using spring balance to weigh it. Swing it around carefully on the end of the string and its weight will increase.

Mass:

This is really just the quantity of matter involved. It normally stays the same regardless of location, speed, gravitational force or whatever. However a mass if accelerated to near the speed of light its mass may change.

In the metric system, mass is measured in kilograms and grams

To find the mass of something you need a balance with a box of reference masses.

► Page created 10 August 2005, amended 28 July 2023.