Question

which situation describes an object that has no unbalanced force acting on it?
(1) an apple in free fall
(2) a satellite orbiting earth
(3) a hockey puck moving at constant velocity across ice
(4) a laboratory cart moving down a frictionless 30.° incline

Explanation:

• An apple in free - fall has only the force of gravity acting on it (assuming no air resistance), so there is an unbalanced force (gravity).
• A satellite orbiting Earth is constantly changing direction (centripetal acceleration), so there is an unbalanced force (centripetal force).
• A hockey puck moving at constant velocity has no acceleration. According to Newton's first law \(F = ma\), if \(a = 0\), then \(F=0\) (no unbalanced force).
• A laboratory cart moving down a frictionless incline has a component of gravitational force acting along the incline, so there is an unbalanced force.

Answer:

(3) a hockey puck moving at constant velocity across ice