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An X-ray tube gives electrons constant acceleration over a distance of $20\text{ cm}$. If their final speed is $2.0\times 10^7\text{ m/s}$, what are the electrons' acceleration?

I know this equation is relevant:

$$v_\text{final}^2 = v_\text{initial}^2 + 2a(x-x_\text{initial})$$

Also I have that $x = .2\text{ m}$ (after conversion from cm), and $v_\text{final}$ is given.

How do I find $v_\text{initial}$? I need that so I can find acceleration.

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Hi user983246, and welcome to Physics Stack Exchange! We don't require the Physics Forums homework template here, so I've edited the formatting of your question to make it a little easier to parse. –  David Z Oct 28 '12 at 1:49
I see. Thanks for doing that. –  user983246 Oct 28 '12 at 3:58
As David says, your problem doesn't really provide sufficient data. If your book has some answer, it would be useful to calculate the initial $v$ from that :-) –  Waffle's Crazy Peanut Oct 28 '12 at 8:28
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1 Answer

To make this a proper problem, it should have told you what the initial speed was. So you're right to be confused; there isn't any information in the problem from which you could deduce $v_\text{initial}$.

However, I'm almost certain the problem author intended for you to assume the electrons start from rest. Call it "physicist's intuition" or whatever. If you are meant to assume a starting velocity, certainly $v_\text{initial} = 0$ is the only one you would have any particular reason to choose.

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I thought the electrons would be ejected from a cathode in the X-ray machine so they have essentially zero speed. –  b_jonas Oct 28 '12 at 12:17
Yeah, that was my thought as well. But the problem should have said that. –  David Z Oct 28 '12 at 12:29
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