Question

Two blocks A and B of masses 2 kg and 1 kg
placed together on a frictionless surface. A force
of 3 N acts on the block-A. The Normal force by
the floor on A is............N​

Answer 1

There are other forces going on but the only force that you need to focus on is the force that A applies on the floor.


Because only then will the floor apply a normal reaction to it. In this case the block A applies a force of Ma*g [Ma is mass of block A] due to its weight and the floor applies a normal reaction which is equal to Ma*g and in opposite direction therefore the normal reaction by the floor on A is


Ma*g
=2*10=20 N