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Homework Help: Can someone clarify this please? Confusing Wording

  1. Feb 25, 2008 #1
    Can someone clarify this please? Confusing Wording....

    1.Two blocks, each with a mass 0.40 {\rm kg}, can slide without friction on a horizontal surface. Initially, block 1 is in motion with a speed v = 1.2 {\rm m/s}; block 2 is at rest. When block 1 collides with block 2, a spring bumper on block 1 is compressed. Maximum compression of the spring occurs when the two blocks move with the same speed, v/2 = 0.60 {\rm m/s}.

    2.If the maximum compression of the spring is 1.9 {\rm cm}, what is its force constant?

    3. Hi everyone, All I really need is clarification of the last given. It says ".....blocks...move with the same speed, v/2 = 0.60 m/s"

    V/2 ? Does this just mean they are both now traveling .60m/s or is there something else I can derive from this that I am missing.

  2. jcsd
  3. Feb 25, 2008 #2
    The actual value you would have to work out, but you can see that if the second block moves faster then the spring loses ground, and if block 2 moves slower then block 1 will push block 2 such that the springs restorative force and all that forces the two into the same speed.
  4. Feb 25, 2008 #3
    I'm sorry...I just don't follow. If block 2 moves slower (stopped) then block 1, block 1 will push block 2 so that the springs (restorative force?...is that like potential energy?).

    The way I'm brainstorming it is that say, without the springs the block would have to be moving at a velocity twice that of the 2nd block's potential energy so that they could ultimately move at the same velocity.

    The spring, must also be significantly weaker to absorb the collision and keep both blocks moving.

    Anyway, I feel like I know exactly what to do...if anyone could offer advice that would be nice but what I really want to do is finish it up right now and post it to verify it.

    Thanks Mindscape for the quick reply...
  5. Feb 26, 2008 #4
    Yes, I think that as you work it out it will start to make sense. Look out for units!
  6. Feb 26, 2008 #5
    I Really hope someone is up and dying to do some physics problems!

    Hi guys, I'm the original poster...

    I'm pretty much just staring at the problem and have been for quite some time...Can someone private message me or e-mail or reply here and help me out with some details...I'm going in circles and really don't want to give up. It's been HOURS!!!

  7. Feb 26, 2008 #6


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    use conservation laws

    Use conservation laws whenever you can.

    In this case, the first step is to use conservation of momentum: the blocks are identical, and they're moving in only one dimension, so we know that the total speed will be constant.
    The total speed initially is v + 0 = v.
    Later, it's v/2 + v/2 = v.

    (Don't bother to work out why - the question has saved you the trouble of doing that by telling you the speeds - I guess this question is mainly to test your knowledge of springs, so the other bits of the question are made as easy as possible for you - sometimes examiners are nice guys!)

    The next step is to use conservation of energy - the total energy of the blocks is less than it was - where has the extra energy gone, and how much is it - that should tell you all you need to know about the spring!

    I hope this has put a spring in your steps! :smile:
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