According to energy conservation ;- Total energy of a body is always conserved, i.e (Initial energy = Final energy)
When A body is thrown up with some velocity(V m/s) This body(m) looses it's K.E.as it go up it's velocity vanishes and gains the P.E.,Hence at the top K.E of the body will fully converted into the P.E.
If E₁ = Initial energy =
and E₂ = Final energy of the body
And, Initial K.E = 1/2(mv²) = 1/2(4×30²) = 1800 J
Final K.E or K.E at the top is Zero as it's velocity is zero at the top.
A/c to energy conservation,
E₁ = E₂
Initial K.E + Initial P.E = Final K.E + Final P.E
or, 1/2(mv²) + 0 = 0 + Final P.E
Final P.E = 1/2(4×30²)
Therefore P.E at the top = 1800 J
And K.E at the top is Zero