Table of Contents

A Guide to Microgravity for All Ages

What is Gravity?

Gravity is a force. A force is a push or pull which if applied to a free body results in an acceleration of the body. Forces are created in many different ways. Some forces require contact; when you push open a door, there is no force on the door unless and until you touch it. Other forces can act without contact, such as electrostatic forces which are associated with the presence of charged particles. Gravity is the force associated with mass and can act without contact.

The earth's huge mass exerts a gravitational force which pulls us down. This force is the reason things fall towards the earth. Actually any two bodies having mass exert a gravitational force on one another, but most pairs of objects have so small a combined mass that the force between them is very small and has no commonly observable effect. That is why when you walk past a car or a building, you don't feel a pull. Very large quantities of mass generate very small forces. For example, two 45,000 kg trucks about three meters apart, exert on each other one tenth the force of a feather sitting in your hand.

Gravitational force between two objects increases as the product of the two masses but decreases with the square of the distance between them. Given two objects a certain distance apart, doubling the mass of one will double the force. Doubling the distance will reduce the force to one quarter of its previous value.

An example of the force of gravity is the attraction which occurs between the earth and moon. The moon's mass causes the gravitational force which pulls on the oceans to create tides and the earth's mass causes the force which keeps the moon in orbit around the earth.

A natural question is: if the earth exerts a force on us and forces cause acceleration, why aren't we accelerating? The reason is that when you are standing on the ground there are two forces acting on you. The first is the force of gravity, and the second is an opposite resistive force acting up on you from the ground. When you add these two forces, the net force is zero and you don't move.