A 2kg stone is projected upward from the ground with a speed of 40m/s.what will be the kinetic energy of the stone after 2 seconds.
what was kinetic energy at ground level?
Ke = (1/2) m v^2 = (1/2)(2)(40^2) = 1600 Joules
how high is it?
v = 40 - 9.81 t
h = 40 t - 4.9 t^2 = 80 - 4.9 (4) = 60.4 meters high
so gain in potential energy = m g h = 2*9.81*60.4
= 1185 Joules
so remaining Ke = 1600 - 1185 = 415 Joules