6. Motion of a Block on a Horizontal Smooth Surface
Case (A) :
When subjected to a horizontal pull :
The distribution of forces on the body are shown.

As there is no motion along vertical direction, hence, R = mg. For, horizontal motion F = ma or a = F/m.
Case (B) :
When subjected to a pull acting at an angle (θ) to the horizontal :
Now F has to be resolved into two components,

F cos θ along the horizontal and F sin θ along the vertical direction.
For no motion along the vertical direction,
we have R + F sin θ = mg
or R = mg – F sin θ
Note:
Hence R ≠ mg . R < mg
For horizontal motion
F cos θ = ma,

Case(C) :
When the block is subjected to a push acting at an angle θ to the horizontal : (downward)
The force equation in this case
R = mg + F sin θ

Note : R ≠ mg, R > mg
For horizontal motion
F cos θ = ma,