A metal block is sliding along an infinite, frictionless, horizontal plane. There are only two forces acting on the metal block: gravity and an upward force from the plane. Which of the following best describes the metal block's motion?

Question

A.
Gravity will accelerate the metal block downward.
B.
The metal block will eventually slow down and stop.
C.
The metal block will move at the same speed forever.
D.
The upward force will accelerate the metal block forward.

Answer 1

The best choice that describes the metal block's motion is:

C. The metal block will move at the same speed forever.

In this scenario, since the plane is frictionless, there are no horizontal forces acting on the block to change its velocity. According to Newton's first law of motion, an object in motion will remain in motion with the same speed and in the same direction unless acted on by an external force. The only forces acting on the block are gravity (downward) and the normal (upward) force from the plane, which cancel each other out in the vertical direction. Thus, the block will continue to slide at a constant speed indefinitely.