Inleastic collision with unknown mass

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Homework Statement


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.

Homework Equations


M1V01+M1V02=M1Vf1+M2Vf2
M1(Vf1-V01)/V02-Vf2=M2

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?
 
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Answers and Replies

  • #2
D H
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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.
That doesn't sound right. The two objects should have the same final velocity if they stick together as a result of the collision.
 
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It does but the information was given from two objects moving on an air track passing through a photogate.
 
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D H
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Please write the problem as it appears in your textbook.
 
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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.
 
  • #6
gneill
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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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