Question

A rock is thrown vertically upward with initial speed $v_0$. Assume a friction force proportional to $-v$, where $v$ is the velocity of the rock and neglect the buoyant force exerted by air. Which of the following is correct?

• A. The acceleration of the rock is always equal to $g$
• B. The acceleration of the rock is always equal to $g$ only at the top of the flight
• C. The acceleration of the rock is always less than $g$
• D. The speed of the rock upon return to its starting point is $v_0$
• E. The rock can attain a terminal speed greater than $v_0$ before it returns to its starting point

Answer 1

The correct option is B The acceleration of the rock is always equal to $g$ only at the top of the flight

Acceleration of stone $=g+kv$ when stone is going upward direction and $g-kv$ ($k$ is constant)when stone is coming down. So acceleration of the stone at top is equal to $g$ as $v$ at top is $0$.
Friction force always opposes the velocity so speed would not increase the terminal speed.