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2014-12-30T13:48:24+05:30
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

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2014-12-30T13:58:09+05:30
K.E.= 1/2*M*V²
      =1/2*4*900
      =1800J
      =1.8KJ
P.E=MgH
      =4*10*45 (H can be calculated by III equation of motion)
      =1800J
      =1.8KJ
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