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Reaction Force

If you place an object on a table you may think no force is acting on it other that gravity - but there are two forces acting (remember - there is always an interaction pair of forces - they never act alone).

Gravity makes the force of its weight act vertically downwards. It will go from the centre of gravity of the object.

But there is also an interaction between an object and the surface it is resting on - so the object pushes down on the surface, and the surface pushes up on the object with an equal but opposite force, and this is called the normal contact force.

A 'normal' is a mathematical line at right angles to the 'tangent' to a surface.

It is the reaction force of the weight - the other part of the interaction pair..