An elevator containing a man weighing 800 N is rising at a constant speed. The force exerted by the man on the floor of the elevator is

An elevator containing a man weighing 800 N is rising at a constant speed. The force exerted by the man on the floor of the elevator is

• A. less than 80 N
• B. 800 N
• C. between 80 and 800 N
• D. more than 800 N

Answer: (B)

The key idea is vertical equilibrium. Since the elevator rises at a constant speed, its acceleration is zero, so the man is not accelerating. The floor must push up on the man with a force equal to his weight to balance gravity, which is 800 N. By Newton’s third law, the force the man exerts on the floor is equal in magnitude to the floor’s push on him, so he pushes downward on the floor with 800 N. The magnitude of that force is 800 N.

The key idea is vertical equilibrium. Since the elevator rises at a constant speed, its acceleration is zero, so the man is not accelerating. The floor must push up on the man with a force equal to his weight to balance gravity, which is 800 N. By Newton’s third law, the force the man exerts on the floor is equal in magnitude to the floor’s push on him, so he pushes downward on the floor with 800 N. The magnitude of that force is 800 N.