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Dumb Dad Question: physics homework

Discussion in 'Fred's House of Pancakes' started by SageBrush, Oct 20, 2008.

  1. SageBrush

    SageBrush Senior Member

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    I asked my son to figure out power consumption of a car travelling on a horizontal at constant acceleration without friction. We know the correct answer is Force * average_velocity, but would like help understanding why the following gives the wrong answer:

    initial velocity and position are zero --
    P - power
    W - work
    F - force
    d - distance
    a - acceleration
    t- time
    v- velocity

    P = W/t = F*d/t = F * a * t^2/2t = F*v/2

    ---
    All we did is subsitute distance as at^2/2, and v/t for 'a' ...
     
    #1 SageBrush, Oct 20, 2008
  2. Rokeby

    Rokeby Member

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    It's a trick question: if there's no friction between the wheel and the road,
    there can be no acceleration. :eek:

    The car did, does, and will forever have the constant velocity of zero. :D

    Newton's First Law, IIRC
     
    #2 Rokeby, Oct 20, 2008
  3. hyo silver

    hyo silver Awaaaaay

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    Unless it's a rocket car. :)
     
    #3 hyo silver, Oct 20, 2008
  4. SageBrush

    SageBrush Senior Member

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    Not meant to be a trick question, believe me.
     
    #4 SageBrush, Oct 20, 2008
  5. brick

    brick Active Member

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    F=m*a . It fixes almost everything.

    **EDIT**

    I think you need to back up to the beginning. What is given in the problem statement?
     
    #5 brick, Oct 20, 2008
  6. kevinwhite

    kevinwhite Active Member

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    Why do you say it is the wrong answer?

    v/2 is the average velocity so the result is Force times the average velocity as you originally stated.

    kevin

    PS I assume that your term "without friction" means without losses caused by air resistance, rolling resistance etc, not that there is no friction between the tire and the road.
     
    #6 kevinwhite, Oct 20, 2008
  7. SageBrush

    SageBrush Senior Member

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    Hi Kevin,

    I cannot convince myself that v/2 is average velocity. Seems like it should be v(final)/t ? Help please
     
    #7 SageBrush, Oct 20, 2008
  8. donee

    donee New Member

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    Hi Sage,

    I think your problem is that a is Delta-v/Delta-t, not v/t. You are confusing the two. So, the equivalence you use is only valid if v0 is (starting velocity) zero, and startint time t0, is zero. In which case v/2 is the average velocity, if accelleration is constant, and v is the final velocity (speed actually, since velocity is a vector quantity).

    In otherwords your final equation should be:

    P = F*(v1-v0)/2 , assuming t0 is zero.

    since a = (v1-v0)/(t1-t0)
     
    #8 donee, Oct 20, 2008
  9. SageBrush

    SageBrush Senior Member

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    Thanks everybody, I finally get it.

    v/2 is the average speed, not v/t. I think it was solving SAT questions ad nauseum with my kids that turned me into a block of cheese. You know -- average is Sum of values/number of elements. It can still be calculated this way, but the numerator is vt/2 and the denominator is t.
     
    #9 SageBrush, Oct 21, 2008
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