# Power of a cyclist physics problem

1. Mar 6, 2008

### jk5389

1. The problem statement, all variables and given/known data
The power of a cyclist moving at a constant speed of 7.0 m/s on a level road is 120 W. What is the magnitude of the frictional force (in N) exerted on her (and the bicycle) by the air?

2. Relevant equations
P=energy/time KE=1/2mv^2

3. The attempt at a solution
Idk i tried but i'm just missing too many variables.. i attempted and got 4.9N

2. Mar 6, 2008

### rock.freak667

Power is the rate at which work,W, is done i.e. $P=\frac{d}{dt}(W)$

W=Fs => (you can take it from here) [Note: constant velocity means constant acceleration which means what about the force and hence you should have a new expression for P]

3. Mar 6, 2008

### dynamicsolo

Here are two things to consider. The cyclist is working along to propel the bike, so they must be exerting a force, yet they are moving at constant speed. What is the acceleration of the cyclist and bicycle? What is the net force of the cyclist and bicycle? What must be the power being delivered by the air resistance against the cyclist and bike?

Knowing the power brought to bear by the air, you can take apart the definition of power, which is P = energy/time or work/time. What is the definition of work and how do you calculate it? Work can be written as a product of something, so it can be factored. What would be a way to factor the quotient for power in terms of a quantity you know from the problem and a quantity you want to find?

Last edited: Mar 6, 2008
4. Mar 6, 2008

### dynamicsolo

Erm, but not just any constant -- exactly what acceleration?