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Homework Help: Accelerated mass and spring system

  1. Feb 25, 2008 #1
    1. The problem statement, all variables and given/known data

    Imagine a mass attached to a spring, equivalent to a vehicle suspension system, where the mass we are considering is the unsprung mass of the wheel/axle assembly. We are told the unsprung mass is given an instantaneous acceleration of 50m/s². We are given the spring constant of the suspension springs (k = 500kN/mm) and we are given the mass of the wheel/axle assembly (m = 200kg). The system is assumed to be undamped.

    I am trying to find the force which is exerted by the spring upward onto the vehicle body.

    2. Relevant equations

    This is directly proportional to the compression of the spring but as I see it I just cannot make it work. Conservation of energy seems best suited, however I require the velocity of the wheel assembly (for ke = ½mv²) which I do not know. I have no time step increment with which to work out the velocity either.

    3. The attempt at a solution

    The acceleration of the wheel/axle assembly is specified in a load case. The problem as I see it is relating this acceleration to the deceleration caused by the spring. I am hopefully just missing something but this problem really has me stuck. If the spring weren’t there then the force on the vehicle body would be a simple F = ma where the acceleration is 50m/s² I think, but this is a much worse than real case.

    The mass is accelerated (say +ve direction) at the 'start' of the action and is immediately deccelerated (-ve direction) by the spring. But how this relationship is expressed I can't fathom.

    Any advice or views are appreciated, thank you.
  2. jcsd
  3. Feb 25, 2008 #2


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    Gold Member

    What is the effect of an 'instantaneous' acceleration ? I don't what it means. Is it a constant acceleration that begins instantaneously ? If so the problem makes sense.
    If it is applied for an instant, there won't be any effect.

    I thnk you have to go with a constant force, which begins abruptly.
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