Free falling objects

Question

A stone is dropped from the top of a cliff. It is seen to

hit the ground below after 3.55 s. How high is the cliff?

Answer 1

The appropriate equation of motion to use would be y = v_it + 1/2at² where "y" is the height, v_i is the initial velocity (which in this case is zero since the stone was dropped not thrown), "t" is the time it was in the air (3.55 s), and "a" is acceleration (in this case -9.8 m/s²). So, plugging in the numbers:

y = (0)(3.55) + 1/2(-9.8)(3.55²)

y = 0 - 61.75225

y = -61.75225 m

The negative sign just means the ground is below where you dropped it (a negative direction). I'll let you round to the appropriate number of sig figs.