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.