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My heads spinning help

  1. Oct 31, 2003 #1
    My heads spinning help!!

    As usual ive left my work to the end of the holiday and after 2 hours of physics questions my head is in a spin... i have a cou[ple of simple quetions where i know the soluion isobvious but i cant get my had around the answers... would anyone be able to point me in the right direction so i can answer them?

    Changes in exponential:
    1) how wold i go about working out the probability of decay of a substance in 1 second.I have been given a half life and a decay rate.

    Spring constants:
    2) I have been given the mass of a spring and the time for oscillations --> how do i work out the spring constant? I know f=kx but i am really confused

    Thanks :)
  2. jcsd
  3. Oct 31, 2003 #2
    Re: My heads spinning help!!

    I'm not sure what you mean by a decay rate. Decay decreases exponentially with time, so it is not constant. Unless your decay rate was given in terms of t^2.

    Anyway, if a sample has a half life of an hour, then an individual nucleus of the sample will have a 50% chance (or 0.5) of decaying in that time. All you need to do then is change that two hours to one second and you have the answer.

    50% in two hours
    25% in 60 minutes
    25/60= 5/12% chance of decaying in one minute
    divide by 60 again for probability in one second.

    Hope that helps
  4. Oct 31, 2003 #3
    Sorry for the double post, but just wanted to say that the two hour half life I used as an example was just invented and you should replace that with the half life they gave you in the question and get down to one second in a similar manner to me.
  5. Oct 31, 2003 #4
    Re: My heads spinning help!!

    I am confused, too. Are you sure you have been given the mass of the spring; or is it a mass hanging from a spring. The latter is the usual type of problem dealing with idealized springs. Look up period of harmonic motion in your text. You will find

    T = 2<pi>(m/k)^(1/2)

    Feel free to visit my website for more help.
  6. Oct 31, 2003 #5


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    Homework Helper

    Re: My heads spinning help!!

    It is not clear what is meant by the "probability of decay of a substance." Do you mean, "probability that a decay will occur in a substance composed of constituents that are prone to decay?"

    You probably mean mass on the end of the spring. There is probably a formula in your book for this. Oh wait. Based on the two questions you asked which don't seem to have anything to do with each other, you're probably using the worst intro phys book I've ever seen. I don't remeber the author or title, but the ones that I have seen have a picture of a waterfall or something like that on the front. It gives extremely brief treatments of QM, thermo, relativity, and such in an extremely oversimplified manner, with utter disconnectedness. If that is the book you are using, I feel sorry for you.
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