A rock is thrown straight up. What is the net external force acting on the rock when it is at the top of its trajectory?

Newton’s second law of motion tells that the acceleration of a system is directly proportional to the net external force acting on the system and is inversely proportional to the mass of the system. Mathematically,

$F_{net}=ma$

Hererole="math" localid="1654167959534" ${}^{F_{net}}$is the net force,${}^m$is the mass, and${}^a$is the acceleration.

When the rock is thrown up with some velocity against the force of gravity, the velocity becomes zero at the top of the trajectory. In the next instant, it starts falling due to the gravitational force acting on the body. When the velocity is zero at the top of the trajectory, the net force acting on the body is zero for a very short period of time. But in the next instant, as the rock begins to fall, the net force acting on the body is the gravitational force.

Hence, the net force acting on the body is the gravitational force for a larger duration of time.