Question

What is the kinetic energy of a 4 kg rock falling through the air at 5 m/s?

Answer 1

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)