Question: what causes a force to be unbalanced?

Susan Cartwright answered on 16 Jun 2015:

I don’t really understand what you are trying to ask. If I kick a ball, I apply a force to it, and it accelerates, because there is a net force on it. If the ball was against a wall when I kicked it, it wouldn’t go anywhere, because the force I apply is now balanced by a reaction force from the wall (in fact, it will probably bounce away, because the two forces acting in opposite directions will squash it, and when I take my foot away the resulting outward force as it expands again produces a net force away from the wall.

So there’s nothing special about unbalanced forces. However, if the object on which the forces act is not accelerating, e.g. me, sitting on this chair, then we know the net force is zero, and so all the forces acting on me must be balanced overall (gravity, acting downwards, and the reaction force from the chair I’m sitting on, acting upwards).

Pairs of contact forces are always balanced: if I push against my desk, the force I exert on the desk is equal and opposite to the force the desk exerts on me. But that’s a special case – in general, there’s no reason for forces to be balanced.