I recently learned how to draw free body diagrams. Suppose a block is kept on ground and we need to draw FBD of ground. One force would be reaction force by block on ground. Will we also include the gravitational force on ground by block $(= mg )$ as a reaction force to gravitational pull on box by earth?
Yes, when considering the forces acting on the ground, in theory you should include both
- the downwards normal force by the block and
- the upwards gravitational force by the block.
In fact the gravitational force felt by the earth due to the object is exactly equal to the gravitational force felt by the object due to earth, just opposite. This Is newton's m 3rd law. The influence in the object is just much greater (it reaches higher acceleration during a fall e.g.) due to its much smaller mass.
When a block is put on the ground. The gravitaional force of earth attracts the block with force $mg$ (More precisely, the centre of earth attracts the block and vice-versa). As a result the ground gets pushed and the ground exerts normal reaction on the block ($N$).
So in $y$ direction there are 2 forces on block -
i) Gravitational force by the earth ($mg$) in $-y$ direction.
ii) Normal reaction by ground ($N$) in $+y$ direction
As there is no vertical motion of block, so $N=mg$