Instant Velocity Question - a falling brick?

Question

A brick is dropped from rest from the roof of a building. On the way down it passes a 6-ft-high window and is observed to pass from top to bottom of the window in 0.25 s. How fast was it moving when it passed the top of the window? The answer is 20 ft/s.

My question is, how do you get 20 ft/s?

Thanks!

Answer 1

If you take the top of the window as the initial point, the equation of motion from that point is

h = v0*t + 0.5*g*t^2  solve for v0

v0 = h/t - 0.5*g*t

v0 = 24 - 0.5*32*.25

v0 = 24 - 4

v0 = 20 f/s

Answer 2

They answered the question correctly, but you need to know in dealing with physics, you have to understand the concepts, and kinematics you need to knot that any equation can be solved with a minimum of 3 variables. If you think you only have two you have to look, usually it's in your velocity. Because you have Vo initial and Vf final one of those is usually equal to zero. or if it's in free fall it's the acceleration due to gravity. You need to really think about it.