What is Newton's second law of motion?

• A. F = ma
• B. F = mv
• C. v = at
• D. p = mv

Answer: (A)

Newton's second law describes how force, mass, and acceleration relate: the net force acting on an object equals the rate at which its momentum changes. When the mass stays constant, this becomes F = ma, linking the force you apply directly to how quickly the object speeds up in a given direction. The bigger the net force, the bigger the acceleration; the heavier the object, the smaller the acceleration for the same force. Mass is the quantity that sets how much acceleration results from a given push, and the acceleration points in the same direction as the net force.

Think of momentum p = mv as the blend of how much stuff there is (mass) and how fast it’s moving (velocity). The law really connects force to how momentum changes over time, dp/dt. If mass doesn’t change, dp/dt reduces to m·a, giving F = ma. That’s why F = ma is the standard form you’ll use in most problems.

The other expressions don’t describe this relationship. v = at is a velocity-time relation for constant acceleration, a kinematic result, not a force–response relation. p = mv states momentum, not force.