1 Newton gives a body of mass 1 kg an acceleration of 1 m.s -2 . This is about the origin of contact force. To every action there is an equal and opposite action, i.e. an equal reaction. It would be interesting if this law was not true, because we could sail yachts by the crew blowing on the sails. The farmer and the horse.
Internal forces are one part of a system acting on another part of the system.
Internal forces do not cause motion, they keep a system together. Unbalanced external forces cause a system to accelerate. With this in mind, we should remember The force which causes the motion of a system is the resultant external force. The effect of force through time is the red area under the graph and written as . Draw a simple diagram of the situation.