If the net external force on a body is zero, what is its acceleration?

If the net external force on a body is zero, what is its acceleration?

• A. Its velocity must be zero.
• B. Its acceleration must be nonzero.
• C. Its acceleration must be zero.
• D. Its velocity must be increasing.

Answer: (C)

Zero net external force means there is no unbalanced push to change how the body moves. Acceleration is the rate of change of velocity, and by F_net = m a, if the net force is zero and the mass is finite, the acceleration must be zero. That doesn't force the velocity to be zero; it just means the velocity won’t change. So an object at rest stays at rest, and an object in motion keeps moving at the same speed and in the same direction. A good picture is a puck sliding on frictionless ice: once started, it keeps moving with the same velocity because there’s no net force to alter it.

Zero net external force means there is no unbalanced push to change how the body moves. Acceleration is the rate of change of velocity, and by F_net = m a, if the net force is zero and the mass is finite, the acceleration must be zero. That doesn't force the velocity to be zero; it just means the velocity won’t change. So an object at rest stays at rest, and an object in motion keeps moving at the same speed and in the same direction. A good picture is a puck sliding on frictionless ice: once started, it keeps moving with the same velocity because there’s no net force to alter it.