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Homework Help: Equate Kinetic Friction to Rotational Energy?

  1. Dec 14, 2014 #1
    1. The problem statement, all variables and given/known data
    So a car owner wants to change his current tires and rims to a wider set of tires and rims for increased handling ability. However, the owner does not want to lose any acceleration performance due to the increased friction of having wider tires. To combat this, the owner wants to know how much weight loss is necessary to overcome the increased frictional forces and maintain his current acceleration abilities. Assume a uniform weight distribution for the wheels.

    All variables are held constant except:
    New wheel width
    New wheel weight

    Coefficient of friction is 0.7
    Stock wheel width is 6in. or 15.24cm
    Stock wheel mass is 20lbs

    Create an equation that compares wheel friction at a given width to wheel weight.

    2. Relevant equations
    I think I should use:
    Kinetic energy = 1/2 Mv^2 where M is mass and v is velocity
    Frictional force = coeff. of friction * F(n) where F(n) is the normal force.
    Normal force = mass * gravity

    3. The attempt at a solution
    am having a tough time with this one. I assume that if I can set the friction equation equal to the rotational equation, then I can use that same formula for the new wheel and the old wheel and can compare the forces easily. I just can't figure out how to set the equation up.
  2. jcsd
  3. Dec 14, 2014 #2


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    I'm not sure whether this question is deliberately misleading or just wrong-headed.
    Some questions to consider:
    - what would the performance be if you were to eliminate tyre to road friction completely?
    - In the equation you quote for frictional force, what role does area of contact play?
    - Is road grip merely down to friction?
    - what slows cars down?

    Edit: I should break the last item into two parts:

    - what reduces a vehicle's top speed?
    - what reduces a vehicle's acceleration?
    (and which of those is relevant for 'performance'?)
    Last edited: Dec 14, 2014
  4. Dec 14, 2014 #3

    Stephen Tashi

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    Is that the statement of the problem?

    Or does the problem say you must calculate something to "maintain his current acceleration abilities"?
  5. Dec 15, 2014 #4


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    The problem statement doesn't make a lot of sense to me at all.

    Increasing friction between the tyre and ground usually improves acceleration - so presumably he is worried about increased rolling resistance?
  6. Dec 15, 2014 #5


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    Oh I think I see. The problem assumes that the wheels must slip in order that the engine achieve optimum power output. Therefore increasing friction must be compensated for by reducing the moment of inertia of the wheels so they spin up just as fast. Gosh the problem statement assumes you know a lot about drag racing.
  7. Dec 15, 2014 #6


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    Okay ----
    If he's accelerating through any turn sharp enough to notice friction from the steered/steering action, there's more going on than is easily calculated.
    Since there's no turn radius specified, take a straight line drive, and there's no difference other than from toe-in and camber which are also not specified.
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