4 A girl on a sledge starts, with a speed of \(5 \mathrm {~m} \mathrm {~s} ^ { - 1 }\), at the top of a slope of length 100 m which is at an angle of \(20 ^ { \circ }\) to the horizontal. The sledge slides directly down the slope.

1. Given that there is no resistance to the sledge's motion, find the speed of the sledge at the bottom of the slope.
2. It is given instead that the sledge experiences a resistance to motion such that the total work done against the resistance is 8500 J , and the speed of the sledge at the bottom of the slope is \(21 \mathrm {~ms} ^ { - 1 }\). Find the total mass of the girl and the sledge.