GPE=81000J or 81kJ
Explanation
Potential Energy = mgh = 20 x 9.8 x ?
To find H use one of the equation of motion
= [(90)^2 - 0 ] / 2(9.8)
Potential Energy = mgh = 20 x 9.8 x 8100 /2(9.8) = 81000 J
Explanation :
It is given that,
Mass of the rock, m = 20 kg
Initially the rock is at rest, u = 0
Final velocity of the rock is, v = 90 m/s
Gravitational potential energy is given by :
[tex]U=mgh[/tex]............(1)
h is the height.
Using third equation of motion :
[tex]v^2-u^2=2ah[/tex]
or
[tex]h=\dfrac{v^2}{2g}[/tex]
Put the value of h in equation (1)
[tex]U=mg\times \dfrac{v^2}{2g}[/tex]
[tex]U=\dfrac{mv^2}{2}[/tex]
[tex]U=\dfrac{20\ kg\times (90\ m/s)^2}{2}[/tex]
[tex]U=81000\ J[/tex]
or
U = 81 kJ
Hence, this is the required solution.
