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Homework Help: Inleastic collision with unknown mass

  1. Oct 21, 2013 #1
    1. The problem statement, all variables and given/known data
    Two objects collide on a horizontal frictionless surface. Object 1 comes from the left with mass .753kg and initial velocity of -.28m/s collides with object 2 moving from the right with an unknown mass and an initial velocity of .28m/s. They stick together when collided and moved to the right. The final velocity of object 1 is .2121m/s and velocity of object 2 is .292m/s. Find mass of object 2.

    2. Relevant equations

    3. The attempt at a solution
    I have attempted this and got an answer of 30.879 as the mass but the correct answer was .648kg. I'm not sure which step I'm doing wrong because I have gone over several times and see if it was just algebra mistake but it wasn't and still got a really big number. Could someone give some pointers what I'm doing wrong?
    Last edited: Oct 21, 2013
  2. jcsd
  3. Oct 21, 2013 #2

    D H

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    Staff Emeritus
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    That doesn't sound right. The two objects should have the same final velocity if they stick together as a result of the collision.
  4. Oct 21, 2013 #3
    It does but the information was given from two objects moving on an air track passing through a photogate.
  5. Oct 21, 2013 #4

    D H

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    Staff Emeritus
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    Please write the problem as it appears in your textbook.
  6. Oct 21, 2013 #5
    This is actually coming from a lab I did a few days ago so it's not in any textbooks... So that's why I'm really confused about this.
  7. Oct 21, 2013 #6


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    Staff: Mentor

    According to your description an object moving from left to right has a negative velocity, and an object moving from right to left has a positive velocity. That's fine; you are free to choose your axes any way you wish. So +x is in the leftward direction.

    But then you say that the final velocities of the objects are positive yet they are moving to the right. How can that be when objects moving to the right should have a negative velocity?

    A collision can be inelastic without the bodies sticking together. When they stick it's a Perfectly Inelastic collision. The imperfect version deals with coefficients of restitution to model the amount of "bounce" that occurs in the collision. Somehow I doubt that this is what's intended here.

    Perhaps you'd best describe your lab setup in more detail. Include a diagram if you can. Did the objects actually stick together? If so, by what mechanism?
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