simple physics problem

[Oracle]

I'm totally new to learning physics and was presented with this problem but the answer was never recorded during the lecture so I'm just checking on here if I solved it correctly or what went wrong..

 

 

"On the Monark Bike - What is the force required to determine a power output of 100W when pedaling at 50rpm?"

 

20N = 4kg x 5m/sec

 

100W = 6000J / 60sec

 

 

*just to note 1 revolution of the pedals will move the bike 6m

[Mr Skeptic]

Are we assuming the bike is moving at a constant speed doing 100W worth of friction to cancel the power input? Otherwise, your force may have to vary depending on time. I think your answer will have to be a torque, unless you know the size of the pedals.

[Iggy]

Your answer of 20 N is correct.

 

F = Pt/d

 

F is force in Newtons, P is power in watts (100), t is time (1 sec), and d is distance (5 m). F = 100*1/5 = 20 Newtons.

[swansont]

20N = 4kg x 5m/sec

 

This part is wrong — your units are incorrect. kg-m/sec does not give you Newtons.

[Oracle]

Thanks for the feedback.

 

This part is wrong — your units are incorrect. kg-m/sec does not give you Newtons.

 

How should that part be written?

 

in my notes it only has F=m.a2 (the 2 is a negative power - I don't understand that part)

 

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Merged post follows:

Consecutive posts merged

Are we assuming the bike is moving at a constant speed doing 100W worth of friction to cancel the power input? Otherwise, your force may have to vary depending on time. I think your answer will have to be a torque, unless you know the size of the pedals.

 

Yeah not that much detail, just a constant speed.

Edited February 28, 2010 by Oracle
Consecutive posts merged.

[swansont]

F = ma

 

a is acceleration (in units of m/s^2), which is the rate of change of velocity

[Mr Skeptic]

Yeah, the -2 power would be for the seconds.