Aball is dropped from an 80-m tall building. neglecting air resistance, what will the speed of the ball be when it reaches the ground 4.0 seconds later?

We are given with the initial height of the building of 80 meters and the time where the speed is to be determined in the problem. we use the formula of motion:

y =  vot + 1/2 gt^2 
vo is zero since free fall
y = 78.48 m

we use another kinematic formula: v2 =v02 +2gy
v2 = 2g*78.48
v = v final = 39.24 m/s

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