A block with a weight of 9.0 N is at rest on a horizontal surface. A 1.2 N upward force is applied to the block by means of an attached vertical string. What are the (a) magnitude and (b) direction of the force of the block on the horizontal surface

a)   9 - 1.2 = 7.8 N

b) Since the force exerted by the box is it's weight, it acts in a downward direction. So the box will exert a force downward (perpendicular to the horizontal surface).

Explanation:

A 9N block would exert 9N of normal force on the horizontal plane under normal conditions. But in this case, we have a spring taking away some of the force by applying an upward force on the box.

So the force exerted by the box on the surface would now be:

a)   9 - 1.2 = 7.8 N

b) Since the force exerted by the box is it's weight, it acts in a downward direction. So the box will exert a force downward (perpendicular to the horizontal surface).