A daredevil motorcyclist is planning a crazy stunt. The idea isthat she will run

Question

A daredevil motorcyclist is planning a crazy stunt. The idea isthat she will run a loop-de-loop that's entirely greased such thatthe motorcycle can't accelerate while on the loop. In order thatshe not die, she decides to calculate in advance whether or not herplan will work. (a) If the radius of the loop-de-loop is 10.0meters, then how fast must she be going at the top in order to notfall down? (b) If you assume that the 150. kg bike-and-rider areslowed by a constant force of kinetic friction on the greased loopequal to 100 N, then how fast must she be going at the start of theloop in order to not fall at the top?
For part a I used mg=mv^2/r tofind v. I got that v=9.9m/s which is correct. I don't even knowwhere to begin on part b.
For part a I used mg=mv^2/r tofind v. I got that v=9.9m/s which is correct. I don't even knowwhere to begin on part b.

Explanation / Answer

a. At the top the forces acting on it are If N is the normal force the forces acting down are N+mg=mv2/r For N to just pass we have, mg=mv2/r v=gr v=9.8*10 v=9.89m/s b. Applying the law of conservation of energy we have KEat bottom=PE+frictional energy at the top of the loop 1/2*mv2=mg*2r+fr*r r=10m fr=100N m=150kg Plug in the values to get the right solution as.

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