# Bicyclist coasting down hill (Newton's Laws)

## Homework Statement

"A bicyclist can coast down a 5° hill at a constant 7.0 km/h. Assume the force of friction (air resistance) is proportional to the speed v so that Fair = cv."

(a) Calculate the value of the constant c.
(b) Calculate the average force that must be applied in order to descend the hill at 22 km/h. The mass of the cyclist plus bicycle is 79 kg.

Newton's Laws

## The Attempt at a Solution

Since a=0, Ʃforce must=0.

Fair=Fg

Fg=mgsin(5°)=Fair

c=mgsin(5°)/v

I just can't figure out how to work around not having mass in this problem.

Any help would be greatly appreciated :)

## Answers and Replies

jambaugh
Science Advisor
Gold Member
What do you mean not having mass? You say the mass of the cyclist + bike is 79kg!

PhanthomJay
Science Advisor
Homework Helper
Gold Member
your equation is correct...the mass is given in part b. Now try part b.

I understand that the mass is given in part B. But not having the mass in the initial problem implies that mass is not needed to solve part A. I was hoping that someone could tell me how to solve part A without mass. Sorry for the confusion

PhanthomJay
Science Advisor
Homework Helper
Gold Member
The mass applies to both parts. You can't solve part 'a' numerically without knowing the mass, and you can't solve part 'b' numerically without knowing the numerical result of part 'a'.