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

Answer 1

The best description of the metal block's motion in this scenario is:

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

Since the plane is frictionless, there are no horizontal forces acting on the metal block, which means that according to Newton's first law of motion (the law of inertia), it will continue to move at a constant velocity (which includes moving at the same speed in a straight line) indefinitely, as long as no other forces act on it. Gravity acts downward, and the upward normal force from the plane balances it out, resulting in no net vertical acceleration.