S.33 3-7

a)
$E_p=mgh$
$E_p=77kg\cdot9{,}81\frac{m}{s^2}\cdot10{,}9m=8233{,}533J\approx8200J=8{,}2kJ$
b)
$E_p=mgh$
$E_p=77kg\cdot9{,}81\frac{m}{s^2}\cdot0m=0J$
c)
$0J-8{,}2kJ=-8{,}2kJ$
d)
$E_k=\frac{1}{2}mv^2=\frac{1}{2}\cdot77kg\cdot\left(14\frac{m}{s}\right)^2=7546J\approx7500J=7{,}5kJ$