I am confused as to how to find K.E. in the Activity 11.15 at Page 155 of Work and Energy chapter because we don't know the velocity in those cases. Finding P.E. is easy there. Can anybody help???

 look ,the mass of the object is 20 kg and is dropped from the height of 4m

therefore,at the height of 4m,the displacement would be 0,so the kinetic energy would be 0

at 3m ,the displacement would be 4m-3m=1m

therefore v(Square) - u(square)=2gs

 =v(square) - 0 =2gs

=v(square) = 2gs

so 2gs=2 x 10 x 3=60

as v(square)=2gs ,you can find v for objects at varous heights

thanks very much ruddi444 for your answer. now it's clear to me...

yes sir! a good job from him. i really appreciated your response.

 mention not :)