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Homework Help: Calculating Momentum and Mass after a Collision

  1. Feb 20, 2016 #1
    1. The problem statement, all variables and given/known data
    The problem states that an object moving at a momentum of 800 m/s( calculated by mass of 80kg x 10 m/s) collides with another object. Both objects are moving forward at 5 m/s. What is the mass of the second object? While moving the objects hit another object moving at 5 m/s in the opposite direction. After the collision they all stop. What is the mass of the 3rd object

    2. Relevant equations

    3. The attempt at a solution
    For the first part I did Momentum=Mass X Velocity. Since the mass of the first object is known and momentum is conserved I got:
    m= 800/5
    The mass of the second object is 160kg

    Now for the 3rd part where they hit an object and stop. The mass of the 3rd object would need to be the same as the combined mass of object 1 and 2. I cannot figure out the equation for this nor do I know if I am on the right track with the solution from part 2. Of course if that is wrong then the entire solution would be wrong.
    Thank you in advance for any guidance.
  2. jcsd
  3. Feb 21, 2016 #2


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    2017 Award

    Staff: Mentor

    There are units missing.
    Is that really the mass of the second object? After the first collision, both the first and the second object are moving at 5 m/s.
    You can solve it similarly to the first collision, but I guess just arguing with symmetry should be fine.
  4. Feb 21, 2016 #3


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    Science Advisor

    You mean that the momentum is 800 kg m/s
    m*5 is the momentum of the second object only, not the total momentum of both objects. The total momentum of both objects is 80*5+ m*5= 800. Solve for m.

    The total momentum of the first two masses is, from above 800 kg m/s. Taking the mass of the third object to be M, its momentum is -5M kg m/s so the total momentum, before the collision is 800- 5M kg m/s. After the collision they are stopped so the total momentum is 0.
    800- 5M= 0. Solve for M.
  5. Feb 21, 2016 #4
    Thank you for the help!
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