Free fall: A rock is dropped from the top of a vertical cliff and takes 3.00 s to reach the ground below the cliff. A second rock is thrown vertically from the cliff, and it takes this rock 2.00 s to reach the ground below the cliff from the time it is released. With what velocity was the second rock thrown, assuming no air resistance?
A) 4.76 m/s upward
B) 5.51 m/s downward
C) 12.3 m/s upward
D) 4.76 m/s downward
E) 12.3 m/s downward

Question

Quizplus · Accepted Answer

The time taken by the first rock (3.00 s) can be used to calculate the height of the cliff using the equation $ h = \frac{1}{2}gt^2 $. Then, using the height and the time for the second rock (2.00 s), we can find the initial velocity using $ h = v_i t + \frac{1}{2}gt^2 $. Solving this gives an initial velocity of 12.3 m/s downward.

Free Fall: a Rock Is Dropped from the Top of a Vertical