Let's take the reference of zero when the skier is downhill. The skier is at a height of 100 with respect to the foot of the hill. Recall that the gravitational potential energy for a body with mass m at a height h is given by:
[tex]E_p=mgh[/tex]
By plugging the values of [tex]m=50[/tex] and [tex]h=100[/tex] and taking [tex]g\approx10[/tex] we get:
[tex]E_p=mgh=50\times10\times100=50000joules[/tex]
