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.

  • 2
    $\begingroup$ 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. $\endgroup$ – David Z Oct 28 '12 at 1:49
  • $\begingroup$ 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 :-) $\endgroup$ – Waffle's Crazy Peanut Oct 28 '12 at 8:28

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.

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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.