Question

A ball of mass 8 kg falls from rest from a height of 100 m. Neglecting air resistance,calculate its kinetic energy after falling a distance of 30 m.

Answer 1

As an object falls from rest, its gravitational energy is converted to kinetic energy

G.P.E = K.E = mgh

K.E = (80 Kg)(9.8 m/s²)(30 m)

K.E. = 23,520 J

Answer 2

At h=100m,
ME = KE + PE
ME = 1/2mv^2 + mgh
ME = 1/2m(0)^2 + mgh     Because the ball has no speed at first
ME = mgh = 8 x 9.8 x 100 = 7840 J

At h=70m   (100m-30m)
ME = KE + PE
7840 J = KE + mgh
KE = 7840 J - mgh = 7840 - 8 x 9.8 x 70
KE = 7840 - 5488
KE = 2352 J

Hope this helps