Question

A rock is thrown straight up. At the top of the trajectory, the velocity is momentarily zero. Does this imply that the force acting on the object is zero?

Answer 1

No.

Step-by-step explanation:

• Due to the Universal Law of Gravitation two masses always attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
• This is also true for the system rock-Earth, which means that at any point of its trajectory, the rock is attracted by the Earth with a force.
• Once thrown straight up, assuming that the air resistance is negligible, no other forces act upon the rock.
• So, even when at the highest point of its trajectory the rock momentarily is at rest, there exists a force acting on the object due to the gravity, which causes the rock to start falling down from there.