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Collision Problem, not even sure where to start.

  1. Feb 12, 2006 #1
    Okay, seeing that I'm new, I'll just tell everyone who reads this that I am a freshman in college, second semester, and am in Introductory Physics. Unfortunately, I've never taken a physics course before, and no science period since Junior year in HS.

    My professor posts online quizzes each week, and I am taking my second one. Each one has four questions, 5 points each, I've solved two of them correctly, but I have no idea where to start on this one:

    A 4000 kg van travelling at 24.0 m/s collides with a 2000 kg car that is stopped at a traffic light. Their bumpers become entangled, and they move forward together. What is their speed immediately after the collision?

    The only work I have to show for it are the masses, velocities, and accelerations of each car (which is pretty easy to find). I know we're supposed to show more work, but I am pretty lost as to how to even go about this question. If someone could please just point me in the right direction on where to go with this it would be greatly appreciated. Thank you :smile:
  2. jcsd
  3. Feb 12, 2006 #2


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    Have you learned about momentum conservation?
  4. Feb 12, 2006 #3
    I have not yet in class, but I did just find something in my book about the law, and it is helping me a lot. I'll hopefully get the answer from here. Thank you for helping :smile:
  5. Feb 12, 2006 #4
    Such type of collisions are called perfectly inellastic where the two bodies after collisions start moving together. Here e or the co-efficinet of restitution is 0. It is defined as the ratio of velocity of seperation betwen the two bodies to that of approach. Since velocity of seperation is 0 as the bodies move together after collision, e is 0.
    Anyway the mommentum is conserved here. The initial mommentum is known. Hence the final mommentum is also same. The mass of the final system consists of both van and the car. Their speed after collision is the final speed of the system that you get by dividing the mass of new system consisting of van as well as car to the initial mommetum of the van.
  6. Feb 12, 2006 #5


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  7. Feb 12, 2006 #6
    Well collision is something that need not much teaching as you told. I couldn't see the above posts before this. Anyway it is a must to know the general equation that mi*vi + ni1*vi1 = mf*vf + mf1*vf1.
    Here i meand initial anf f means final.
    In any collision in the absense of external forces the mommentum is conserved. This is directly dealing with the concept that an external force is required to change the mommentum of a body as according to first law. Similar conditions hold even when we consider a system. Some external force is required if we need to change the mommentum of a system due to collision. Remember that the collision is internal force to our two system comprising of the colliding objects.
    I think you might go for solving more problems and then the theory would become clear on its own.
    I give you one question which you have to try.
    Let the masses of two bodies be m and M and travelling with a velocities u and U before collision and their velocities change to v and V after collision. Now it is given that not only mommentum but also Kinetic Energy is conserved. So you have two equations and two unknowns. So it is solvable. Try expressing v and V with respect to m and M as well as u and U
  8. Feb 12, 2006 #7
    Astronuc you are getting older day after day. Is the one in the avator really your future face or present face?
  9. Feb 12, 2006 #8


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    :rofl: Several months ago. But the face is present more or less.
  10. Feb 12, 2006 #9
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