Could you sled backwards down a hill?

A child and sled with a combined mass of 55.0 kg slide down a frictionless hill that is 7.99 m high. If the sled starts from rest, what is i?

• A child and sled with a combined mass of 55.0 kg slide down a frictionless hill that is 7.99 m high. If the sled starts from rest, what is its speed at the bottom of the hill?

• Answer:

  Energy balance Change in energy of the system = 0 change in KE + change in PE = 0 1/2*m*(vf^2 - vi^2) + m*g*(hf-hi) = 0 m's cancel. Starts from rest implies vi = 0. Assume the bottom of the hill is hf = 0. 1/2*vf^2 - g*hi = 0 vf = sqrt(2*g*hi) Plug in numbers vf = sqrt(2*9.81 m/s^2 * 7.99 m) = 12.5 m/s