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Homework Help: Calculating the Force on impact?

  1. Jan 14, 2015 #1
    Hi folks, this is a problem I am having in a collage project I am doing. I'm not too sure about my results and keep chasing my tail using the same formulas, e.g. F=ma etc. I think I need to allow for the fact that the force is applied as a collision but I am not sure how or if I need to do it.
    So the big question, is the force that is applied to the ground to accelerate the bike the same as the force that is applied to the object that it impacts? I have tried to provide all the relevant data, but please ask if you need more.
    Thank you.

    1. The problem statement, all variables and given/known data

    A motorcycles engine provides a torque of 46.91 Nm the transmission multiples this torque to 1053.87 Nm. This torque applied to the ground through a wheel with an outer diameter of 0.72 m. The motorcycles front wheel hits an object at a distance of 1.3 m. Bike and rider mass=194.37 kg.

    Force = torque/distance
    Force = 1053.87/(0.72/2)
    Force applied to the ground to accelerate the motorcycle = 2924.97 N

    Force=mass x acceleration
    a = 2924.97/194.37
    a=15.05 m/s^2

    Final Velocity^2 = Initial Velocity^2 + 2 x acceleration x distance
    V = squrt(0+2x15.05x1.3)
    velocity = 6.14 m/s

    Kinetic Energy = 1/2 x mass x velocity^2
    Ek = 0.5 x 194.37 x 6.14^2
    Kinetic Energy = 3664.67 Joules

    Assuming no losses and that there is an instantaneous transfer of torque to the ground etc.
  2. jcsd
  3. Jan 14, 2015 #2

    Stephen Tashi

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    It isn't clear what you mean by that statement.
  4. Jan 14, 2015 #3
    Sorry,The motorcycle is at rest with the front wheel of the motorcycle at a distance of 1.3 m from the object. The motorcycle then accelerates from rest and collides with the object.
  5. Jan 14, 2015 #4


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    No. Force isn't conserved.
    Momentum is conserved and sometimes energy is conserved depending on the nature of the impact.

    There are several ways to work out what the impact force might be but you need to know more about the nature of the collision. For example if the bike ran into something that caused it to stop in say 0.5 meters you could work out the average force that the object applied to the bike.

    Edit: but would the rider also stop in 0.5 meters :-)
  6. Jan 14, 2015 #5


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    PS: Note that short stopping distances equate to high forces and a zero length stopping distance would imply infinite force. Something has to give.
  7. Jan 14, 2015 #6
    Ok so, if the motorcycle collides with the object and the object is a cantilever, if the collision is with the free end of the cantilever the free end deflection could be considered the distance it takes to stop. Then work out the average force applied to the bike?

    PS: I know first hand that the rider only stops if he meets the object in question! But its ok because there is plenty of friction between ride/road to slow you down!
  8. Jan 14, 2015 #7
    Thank you for the advice I think I have it now
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