WEBVTT
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problem. 8.21. So we have some bucks moving
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around frictionless Lee on an air table. Hey,
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is moving to the right with some speed that we
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don't know where's Bee, which is initially address,
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hear their masses and then sometime later they will collide
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. And then after that okay is moving to the
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left to the speed of 0.12 meters per second.
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And he is moving to the right of the speed
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of 65 meters per cent. And so the two
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things we want to find or what was the initial
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speed of a sort of a big mystery here,
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like, you know, very interesting to know that
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. And then what is the change in the kinetic
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energy? Because it will turn out This vision is
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not the last. It's partially elastic. So let's
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start with party. We know from the conservation of
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momentum that the initial momentum, which since is the
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only thing moving will be just the massive eh times
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. It's eager to you like to know, And
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now this is going to be equal to the sum
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of the Momenta over here. That's a day because
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that has not changed. Piece of a final.
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It should be a negative number because it's moving to
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the lab. Plus, I meant to be now
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, in order to solve this, it's pretty easy
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. You just divide both sides by a and so
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we have the final speed of A. This is
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going to be negative. Plus the ratio of the
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masses, the final speed. And so now we
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just put all of these things in since so bye
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here. But it's numbers. Together you get its
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initial speed 0.7 i zero now to find a change
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in Connecticut, all we have to do this computer
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be a final and initial that again because now we
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know how fast everything was moving in both situations.
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So the final Connecticut take this one outside. You
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say that you did it massive, eh? It's
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finalised, which is naked recall, but they're squaring
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it. Explain. My kinetic energy had to begin
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with you. And so then put in all of
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these numbers we get that change in Medic energy is
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negative, which they asked, zero are naked because
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this is nothing going on in this problem. Externally
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add energy to the system. And so the Connecticut
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you can either be in only either be conserved or
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if the deformation of the pucks is not perfectly elastic
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. So that will turn into, you know,
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very slight temperature rise and the pots. And this
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comes out to be a fairly small number. 0.2
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the jewels. Some energy was lost, but not
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a lot. If you'd like to compute what the
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percentage changes, you divide the final kinetic energy by
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the initial what percentage of the energy was concerned?
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But we don't need to worry too much about that
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. Here. We could do that yourself. Curious
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would be a good good exercise for you, perhaps
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, But this is the second thing we were asked
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if I This was the first thing I asked if
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I So we've solved the mystery of B, and
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we've also determined that this was not an elastic