A rock tumbles straight down from a cliff, it is moving at 20 m/s when it hits. What is the height of the cliff? The mass of the rock is 8 kg. A. 400 m
B. 160 m
C. 2.5 m
D. 20.4 m

D. 20.4m

Explanation:

assuming there is no initial velocity, where v=final velocity, g=acceleration due to gravity, and t=time

v = g*t

20 = 9.8*t

t =2.03 s

y = (1/2) g t^2  where y=vertical distance, g=acceleration due to gravity, and t=time

y = 4.9 (2.03)^2

y = 20.4 meters

the minimum amperage that the current in this wire would have to have so that its weight is completely supported by the magnetic force due to earth's field, assuming that no other forces except gravity act on it