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Finding ratio of impluse

  1. Jul 25, 2015 #1
    • User informed about mandatory use of the homework template.
    Two identical molecules A and B each of mass M travel with the same velocity towards a vertical wall at an angel 20 degrees with respect to the normal to the wall. A rebounds from the wall with the same speed while B sticks to the wall.

    Determine the ratio of the implus experience by A to that by B during collision.

    I know the inpluse experienced by A is
    final momentum - initial momentum=
    Mvcos20-(-mvcos20)

    But for B, why is the impluse mv? And not 0- mvcos20
     
  2. jcsd
  3. Jul 25, 2015 #2

    Orodruin

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    What is the velocity of B before the collision? What is the velocity after? As a result, what is the difference in B's momentum before and after?
     
  4. Jul 25, 2015 #3
    Isn't the velocity of B before collision vcos20. And velocity after is 0 since is stick to wall. Therefore the momentum is 0-mvcos20.
    But the correct answer is mv which I don't understand why
     
  5. Jul 25, 2015 #4

    Orodruin

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    No, that is the velocity component orthogonal to the wall.
     
  6. Jul 25, 2015 #5
    But why for A is vcos20, what's the difference?
     
  7. Jul 25, 2015 #6

    Orodruin

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    A does not stick to the wall. The momentum component of A orthogonal to the wall is simply reflected while that perpendicular to it is unchanged.
     
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