Acceleration of Free Fall

Acceleration of Free Fall

The weight of a body is given by W = mg, where m is the mass, and g = 9.81 m/s² is the acceleration of free fall. Rearranging, g = W/m, which is also the force per unit mass. So g is also the gravitational field strength.

This means that on the Earth's surface, the gravitational field strength is 9.81 N/kg. It is nearly the same everywhere on the surface of the Earth. On top of a high mountain, where it is a bit further than the centre of the Earth, g is a bit smaller. At the north pole and south pole, the sphere that is the Earth is a bit flattened. There, the surfaces are a bit nearer to the centre of the Earth, so g is a bit larger.

The variation of g on the Earth's surface is generally small. When we use W = mg for the weight on the Earth, we normally assume that g is constant anywhere on Earth. (Some people think that this variation in g has made a difference to the records at the Olympics.)

Now that we have learnt Newton's law of gravitation, we know that the weight decreases if we move the body higher up. For everyday purpose, we are seldom more than a few metres above the ground, so g would be very nearly the same.

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