Hello friends! I'm new to this site and can see its good. Well I have a doubt which I would like to get clarified. According to newton's 1st law , Any body continues to be in its state of rest unless and until an external force acts on it. Imagine a box kept on the floor. There are 2 forces acting on it. 1)gravitational force ie downwards. 2) upward thrust by the floor. So according to newton's 3rd law, For every action there is an equal and opposite reaction. How do we know it is equal?What is the proof? Assume that the magnitude of gravitational force is greater,then the box should start moving in the direction of the gravitational force.ie downwards. Am I right? Please help.