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Homework Help: 2 Conveyors with different speeds

  1. Mar 30, 2017 #1
    (mentor note: moved here from a non-homework thread)

    I am trying to find the time it will take for a parcel to pass over one conveyor belt

    The conveyor on the right is Conveyor A The conveyor on the left is Conveyor B
    they both have a length of 700mm
    Conveyor A has a constant speed of X
    Conveyor B has a constant speed of Y
    The hockey pick has a radius of 80 mm
    The coefficient of friction is Z
    the hockey puck is initially moving at speed X

    I need to find the time it will take for the parcel to pass completely over Conveyor B ( shown with arrow in picture).
    Last edited by a moderator: Mar 30, 2017
  2. jcsd
  3. Mar 30, 2017 #2


    Staff: Mentor

    This looks like homework. We cannot solve it for you.

    Please show some work so we can provide some hints to help you work it out.
  4. Mar 30, 2017 #3

    I am actually looking for a way to start as I am not sure how to approach it.

    Here's what I have so far:

    The question I believe lies at the point where the puck crosses from the first conevyor belt to the second.
    I have this force diagram but I am not sure if it's right
  5. Mar 30, 2017 #4


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    You need to consider a more general state during the cross-over. When there is a length x on conveyor B, how much weight will be on that conveyor? You will have to make some reasonable/idealised assumptions to answer this.
  6. Mar 30, 2017 #5
    I generalized the problem to get an approximate answer.
    I removed the conveyor belt on the right and just assumed that the hockey puck has a speed of X when introduced on to the conveyor belt on the left
    So U ( initial speed) = X
    V ( final speed) = Y
    S (distance) = 700
    T (time taken)= T

    I assumed that the acceleration is constant, hence s = (U+V)/2 *T

    Is this approach correct to get an approximate answer ?
  7. Mar 30, 2017 #6


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    No. Imagine making the B conveyor much longer. The distance the puck travels on B before coming to the same speed as B does not keep increasing. It is controlled by the coefficient of friction.
    You could get an approximate answer by just supposing the puck is dropped onto B with the horizontal speed of A and calculating the time it would take to accelerate to the speed of B. But if that were the expected solution I do not see why they would have given you the 9mm separation distance.
  8. Mar 30, 2017 #7
    It's not a homework questions, I am actually working with conveyor belts and need to calculate the speed the conveyor B based on time it takes for the parcel (hockey puck) to pass conveyor B. So this is real life problem not a homework.

    With that said being said. Your second part suggesting calculating the time it would take for the puck to reach speed B, how do I do that ?
    I figured that the puck would instantly have the speed of conveyor B as the puck won't slide and hence it will have a speed of 0 relative to the belt
  9. Mar 30, 2017 #8


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    But in your earlier attempt you assumed a constant acceleration.
    You quote a coefficient of friction. Is that kinetic or static? If it is kinetic, what is the frictional force when sliding? What acceleration does that give?
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