Question

A perfectly still 0.01 kg marble is hit with a force of 50 N by a pencil. According to Newton's third law, which describes the force the marble exerts on the pencil? A. The marble exerts a force of 0.05 N on the pencil. B. The marble exerts a force of 0.5 N on the pencil. C. The marble exerts a force of 5 N on the pencil. D. The marble exerts a force of 50 N on the pencil.

Answer 1

During the time that the pencil is exerting 50N of force on the marble, the marble is exerting 50N of force in exactly the opposite direction on the pencil. (D)  The mass of the marble doesn't matter, and it doesn't even matter whether the marble is moving or perfectly still.

As soon as the pencil stops exerting any force on the marble, the marble immediately stops exerting any force on the pencil.