Question

A stone is thrown directly upward with a speed of 30 m/s. What is its velocity 4 s later? Assume for convenience that g = 10 m/s^2.

A. 10 m/s, upward

B. 10 m/s, downward

C. 70 m/s, upward

D. 70 m/s, downward

Answer 1

Let us assume upward direction as positive and downward direction as negative. Then, it is given that

Initial velocity of stone, u=30m/s

Gravitational force will act in downward direction, and will oppose the upward motion. So, we take the acceleration, a=-10m/s²

Let velocity of stone after t=4 seconds be 'v' m/sec, Then

We know that: v=u+at (first equation of motion),

On substituting the values of u, v, a and t in this equation,

We get v=30m/s-10(4) m/s= -10 m/s

Here the negative sign indicates that the velocity will be in downward direction.

Therefore, the velocity after 4 seconds will be 10m/s in downward direction (Option B).