1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Finding 'a' when 'x' is given as a function of 't'.

  1. Oct 9, 2013 #1
    Hello everyone! I have this question given to me by a friend and the question is:
    Position of a particle x is given by x^2=t^2 + 1, t is time in second, find acceleration for time t.

    So this is what I decide to do:
    x^2 = t^2+1
    x = sqrt(t^2+1)
    if you double differentiate the function on RHS
    You get:
    (t^2+1)^-3/2 or 1/(t^2+1)^3/2

    I'd be grateful if you give me the steps for finding the answer (including the steps for finding out the derivative, because I used an online calculator).
  2. jcsd
  3. Oct 9, 2013 #2
    Are you saying you don't know how to determine the derivative of the right hand side? If not, show us how you get the first derivative?
  4. Oct 9, 2013 #3
    No, I do not know how. :(
  5. Oct 9, 2013 #4
    It's the standard derivation of a function... hint: think how you can express roots as powers and think what the derivative of a power is...
    Good work
  6. Oct 9, 2013 #5
    Have you had calculus yet?
  7. Oct 9, 2013 #6
    Yes; I tried double differentiating it but my answer didn't match the one given in the calc. the answer I got was


    This mistake is all the more reason I want the steps. I know that if you double differentiate 'x' I get 'a' but for some reason by answer does not even come close to the original answer. It'd be really kind of you guys if you could just tell me if I was correct or wrong, and if wrong where I went wrong and how I should proceed further.
  8. Oct 9, 2013 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Have you heard of the chain rule?
  9. Oct 9, 2013 #8
    From this answer, it just looks like all you did was make a mistake in algebra. So us the details so we can see if we can spot it.

  10. Oct 9, 2013 #9
    GUYS GUYS! Forget about the calculus, please tell me if the algorithm I gave for sum is correct or not? If not, tell me the correct algorithm and the solution so I can try it out and match my answer.
  11. Oct 9, 2013 #10


    User Avatar
    Science Advisor
    Homework Helper

    What algorithm?

    How about you try the first derivative first?
    ##x = \sqrt{t^2 + 1}## - what is dx/dt? You are right that the derivative of ##\sqrt{u} = u^{\tfrac12}## is ##\tfrac12 u^{-\tfrac12} = \frac{1}{2 \sqrt{u}}##. But as the square root is over t² + 1, not u, you will need the chain rule like SteamKing said.
  12. Oct 9, 2013 #11
    You didn't give any detail about what you did... haw are we supposed to know where and why and what you did wrong????
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted