If a net horizontal force F acts on a mass m on a frictionless horizontal surface, what is the acceleration?

• A. a = F/m
• B. a = mg
• C. a = F
• D. a = 0

Answer: (A)

Newton's second law says that acceleration equals the net force divided by the mass. On a frictionless horizontal surface, the only horizontal force is the given force F, so the net horizontal force is F. Therefore the acceleration is a = F/m. This aligns with the idea that doubling the force doubles the acceleration, while increasing the mass slows it down. The force F by itself is not acceleration (it has units of newtons), and gravity (mg) acts vertically and is balanced by the normal force, so it doesn’t affect horizontal motion. If there were no net force, the acceleration would be zero, but here there is a net force.