A student throws a 5.0-newton ball straight up.
What is the net force on the ball at its maximum
height?
(1) 0.0 N (3) 5.0 N, down
(2) 5.0 N, up (4) 9.8 N, down
If the ball weighs 5 newtons, then that's the force of gravity that attracts it to the center of the Earth. The force only depends on the mass of the ball and its distance from the center of the Earth. The force of gravity doesn't care whether the ball is moving or at rest. It doesn't change significantly until the ball moves to some significant distance off the Earth's surface, which isn't happening if it's tossed by a human being.
So we have a downward force of 5 newtons on the ball at all times. In order to find the net force on it, we have to identify any other forces besides gravity that are also acting on it.
If the ball were moving through the air, then there would be a force of air resistance on it, acting opposite to its motion. But when it's at its maximum height, in the process of turning around from rising to falling, there's an instant when it's not moving through the air at all, so gravity is the only force acting on it. At that moment, the net force on the ball is 5.0 newtons downward.
