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: How to differentiate.

  1. Dec 5, 2011 #1
    1. The problem statement, all variables and given/known data
    I'm total newbie when it comes to differenciatiation, and was wondering if I'm doing this correct.

    Basically, this is a physics problem, but thought I'd put this in the calculus section for specific help.

    It's about a ball undergoing SHM, after being dropped down a shaft on an asteroid! (an everyday occurance you'll find)

    I've got to find the constants for A, B, and C.

    We haven't been told what they represent, but I'm pretty sure B is velocity, C, acceleration due to gravity (Not Earths gravity, remember) and I think A is either Amplitude, or initial position. I'm not sure.

    2. Relevant equations

    x(t) = A + Bt + Ct^2

    Where the ball is released at x=R at t=0s

    3. The attempt at a solution

    Not sure i'm doing this correctly, but for the first derrivative...

    R(0s) = A + B x 0s + C 0s^2 = A.

    So does this mean, that time at 0s is in position A?
  2. jcsd
  3. Dec 5, 2011 #2
    Yes it does.
  4. Dec 5, 2011 #3
    more importantly it means that A = R
  5. Dec 5, 2011 #4
    Hmm I'm having trouble going in an editing my posts, I apologize that there are multiple ones. You also know that the ball is released from rest at time 0 as well. What can you do with that equation to find some more information?
  6. Dec 5, 2011 #5
    So, I presume A is Initial position, and not the amplitude?

    Anyway, second derivative. Velocity.

    R(t) = Bt + 2Ct^1

    Which is 0 again isn't it? As time = 0s.

    Not sure i've done this correctly.
  7. Dec 5, 2011 #6
    Just realised the question does say "at small times the ball has the x-component of the ball has the following form", so shall I change t to perhaps 0.1s to discover position etc?
  8. Dec 5, 2011 #7
    You didn't. that 't' in 'Bt' should not be there.
  9. Dec 5, 2011 #8
    I must go for the day MrRandom. You will find the method of solving these types of equations by googling "Linear Second Order Differential Equations with Constant Coefficients". Good luck to you.
  10. Dec 5, 2011 #9
    Ok thanks for your help. If someone else could help me I'd appreciate it.
  11. Dec 6, 2011 #10
    I think i've got the differenciation part susses.

    v(t) = dx/dt = B + 2ct
    a(t) = d2x/dt2 = 2c

    Now i', supposed to find numerical values for these, but not sure where to begin.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook