CBSE Class 9 Answered

When we push against a wall, the wall also push us in response i.e. we get a reaction to our action of pushing against the wall. The formula F = ma, where F is the net force acting. Since we are pushing against wall and the wall also i.e. action and reaction, magnitude is same  but the directions are opposite. Let's say we push the wall in +x direction and the wall will push i.e. reaction will be -x direction. And they'll cancel each other. Hence no net force and no acceleration. We conclude that action = reaction. 

To calculate the how much we are pushing, we have to use a spring in between wall and palms of our hand.

The compression in the spring can tell how hard we are pushing.

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