1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Energy and power supplied by a bicyclist

  1. Jan 6, 2017 #1
    1. The problem statement, all variables and given/known data
    In bicycling, each foot pushes on the pedal for half a rotation of the pedal shaft; that foot then rests and the other foot takes over. During each half-cycle, the torque resulting from the force of the active foot is given approximately by τ = τ0 sin ωt, where τ0 is the maximum torque and ω is the angular speed of the pedal shaft (in s-1 as usual). A particular cyclist is turning the pedal shaft at ω=70.0 rpm, and at the same time τ0 is measured at 38.5 N•m.
    Find:
    (a) the energy supplied by the cyclist in one turn of the pedal shaft and
    (b) the cyclist's average power output.

    2. Relevant equations
    Work:
    W = τθ, with θ the angular displacement;
    Work-energy theorem:
    W= ΔKE = Energy supplied by the cyclist

    3. The attempt at a solution
    For (a) I thought of using the work-energy theorem the following way:
    E=τ0 sin(ωt)θ
    with:
    - θ=2π rad=one revolution
    - t=θ/ω
    So I get E = 38.5*sin(ω*2π/ω)*2π = 0 !
    This is not really what I expect....

    For (b) there may be a rotational version of P=F*v that is P=τω.

    Thanks for your help
     
  2. jcsd
  3. Jan 6, 2017 #2

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Welcome to PF!
    Since the torque depends on time, you will need to use calculus.

    Yes, but this would give the instantaneous power. You want the average power.
     
  4. Jan 6, 2017 #3
    Thank you!
    So I apply the following integral from 0 to 2π:
    W=∫τ0 sin(ωt)dθ
    Then :
    dθ/2π = dt/T (T:=total time=2π/ω) ⇒ dθ=ω dt
    ⇒ W = ∫τ0 sin(ωt)/ω dt = -τ0 cos(ωt)/(ω^2)
    Evaluating the integral from 0 to 2π gives 0, again. Or should I integrate from 0 to π for the up-down movement?

    Thanks again for your answer
     
  5. Jan 6, 2017 #4

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Note that the expression τ0 sin(ωt) only holds for half a cycle. (This expression would give a negative torque for the second half of the cycle. But this doesn't happen. Instead, the other foot takes over.)
    How did ω get into the denominator?
    Yes, integrate θ from 0 to π; or integrate t from 0 to the time corresponding to θ = π.
     
  6. Jan 6, 2017 #5
    Very nice, thanks!

    This was a mistake indeed, it's supposed to be in the nominator.

    So just a recap for (a):
    W = ∫τ0 sin(ωt)dθ = -τ0 cos(ωt) → from 0 to π gives → Supplied Energy = W = τ0 cos(ωt)
    We must consider that ω in the cos term has unit s^(-1) so we multiply it by 2π to have it in rad/s.
    Numerically,
    ω=70 rpm = 7.33 rad/s
    t=π/ω=0.429 s
    So E = 38.5*cos(2π 7.33⋅0.429) = 23.68 J.

    So the energy supplied for one turn is:
    E = 47.36 J.

    (b) The average power is the work difference per unit time which leaves:
    P = ΔW/(2*Δt) = 47.36/(2*0.429) = 55.20 W.
    (And here Δt is multiplied by 2 for both feet movements)

    Hopefully this is correct.
     
  7. Jan 6, 2017 #6

    TSny

    User Avatar
    Homework Helper
    Gold Member

    OK
    I don't think you evaluated at the limits correctly. Otherwise, your work for the rest of parts (a) and (b) looks correct to me regarding method.
     
  8. Jan 6, 2017 #7

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Just caught something else.
    Why the factor of 2π? Note that t=π/ω tells you that ωt = π. That's all you need to evaluate cos(ωt).
     
  9. Jan 6, 2017 #8
    Thanks.
    Oh I see... I thought that, as ω is specified to be given in s^(-1), we were expected to convert it into rad/s (so multiplying it by 2π)... But OK I understand it now.

    So actually
    (a) E = 2*38.5 = 77 J.
    (b) P = 89.7 W.
     
  10. Jan 6, 2017 #9

    TSny

    User Avatar
    Homework Helper
    Gold Member

    I think these answers are off by a simple factor due to the fact that you didn't evaluate W = ∫τ0 sin(ωt)dθ = -τ0 cos(ωt) correctly at the lower limit (t = 0).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Energy and power supplied by a bicyclist
  1. Power supplied (Replies: 2)

Loading...