A 2.6 N block is at rest on a horizontal surface. A 0.9 N upward force is applied to the block by means of attached vertical string. (a) What is the magnitude of the force of the block on the horizontal surface?

To find the magnitude of the force of the block on the horizontal surface, we need to consider the equilibrium of forces acting on the block.

In this case, the block is at rest, so the net force acting on it must be zero. This means that the force applied by the string must be balanced by an equal and opposite force exerted by the horizontal surface.

The force applied by the string is 0.9 N upward. According to Newton's third law of motion, the force exerted by the block on the string is equal in magnitude but opposite in direction - 0.9 N downward.

Since the block is at rest, the magnitude of the force exerted by the block on the horizontal surface is equal and opposite to the force exerted by the string on the block. Therefore, the magnitude of the force of the block on the horizontal surface is also 0.9 N.

To summarize:
Magnitude of the force of the block on the horizontal surface = 0.9 N