Question

According to Newton’s second law of motion, when an object is acted on by an unbalanced force, how will that object respond?

Answer 1

It will accelerate. A force of magnitude F exerted on an object of mass m applies an acceleration a according to

F = m a

Remember that force and accleration are vector quantities, so the object's acceleration would point in the same direction as the applied force. The other choices describe some possible outcomes, but

• it the object starts in motion, it can only stop moving if the force opposes the motion and acts in the opposite direction. The object would eventually stop, but only for an instant before changing direction and starting to move again. Consider a ball being thrown directly upward, reaching its highest point, then falling again.

• its speed (which is a scalar quantity) would only decrease if the object starts in motion and is slowed down. But if it starts at rest, its speed can only increase. Consider a ball being dropped from some height and gaining speed as it falls.

• its velocity will certainly change, but can only increase if the object is at rest, or if it's already in motion and the force points in the same direction.