# Force exerted on bicycle to maintain cosntant speed

1. Jan 17, 2012

### ErwinJL

1. The problem statement, all variables and given/known data
A 70-kilogram cyclist develops 210 watts of power while pedaling at a constant velocity of 7 m/s east. What average force is exerted eastward on the bicycle to maintain this constant speed?
m = 70kg
W = 210 J
V = 7 m/s
F = ?

2. Relevant equations

3. The attempt at a solution
The way I did this problem was by looking at my units. I'm trying to find force (newtons) when given mass (kilograms), watts (Joules per second), and velocity (meters per second). I know that watts in joules per second is equal to newtons times meters per second, which in turn, becomes newtons times velocity. In the end, I use watts equals newtons times velocity and solve for newtons, dividing watts by velocity (in this case, 210 watts divided by 7 meters per second). I end up with 30 N. Is this a correct way of doing the problem? Is there another way to do it with an equation and number crunching?

2. Jan 17, 2012

### Dick

That is perfectly correct. Work=Force*Distance, so dividing by unit time gives you Power=Force*Velocity. But that's pretty much what you have already worked out using the units. Paying attention to units is a great thing and you are doing that well.

3. Jan 17, 2012

### Staff: Mentor

Looks like the mass of the cyclist, here, is just a red herring.