$4\mathrm{.}2\mathrm{.}39.[$Level $2]$ Let's see what happens to a skateboard rider as she starts down the track. Treat the $70$ lb rider as a mass particle that slides down the track in a friction$-$free manner. Use conservation of energy to determine her velocity when $=45$ and then apply a force balance using polar coordinates to determine the normal force between the skater and the track. Comment on the result.