Answer:
50j
Step-by-step explanation:
Let’s review the 4 basic kinematic equations of motion for constant acceleration (this is a lesson, so commit these to memory! They will serve you well.):
s = ut + ½ at^2 …. (1)
v^2 = u^2 + 2as …. (2)
v = u + at …. (3)
s = (u + v)t/2 …. (4)
where s is distance, u is initial velocity v is final velocity a is acceleration and t is time.
In this case, we know u = 0, g = 9.81m/s^2, s = 50m
So, using equation (2), v^2 = 0 + 2(9.81)(50) = 981(m/s)^2
Then v = √981 = 31.32m/s
So, saying that the velocity is 5m/s is totally incorrect.
However, if the intent of this question is to demonstrate the calculation of kinetic energy, we could say KE = ½mv^2,
So KE = ½(4kg)(5m/s)^2 = 50J
(But the real answer is 2(981) = 1962J)