Question

If Newton's third law says that for every action there is equal and opposite reaction.But when a truck is at rest why does it doesn't move?

Answer 1

A truck, like any object, exerts its weight down on the surface it is on. The truck at rest, thus exerts a force on the ground, (mg) which is the weight. However, at the same time, the ground exerts a force of reaction back on the truck with whichever part of the truck it is in contact with. Therefore, if there are four wheels on a truck, all four experience a reaction force. If we assume the truck to be on a flat horizontal plane, then the reaction force is equal to the force exerted by the truck(Newton's third law.)
If you still don't understand, imagine you are standing straight, and you push a wall. The walk does not move. But you still feel a force underneath your palms.that is the reaction for if the wall on you.

Answer 2

is if the reaction force is greater than the action force displacement takes place???