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: The velocity of a particle at the origin.

  1. Sep 25, 2012 #1
    1. The problem statement, all variables and given/known data
    A particle moves according to the position function x(t)=ct2+bt where c=3m/s2 and b=-7m/s. Find the velocity at the origin.

    2. The attempt at a solution
    I tried just taking the derivative and setting t=0 to the equation as so:
    vx(t)=6t-7=6(0)-7=-7 m/s
    Although, when I put the answer into WebAssign it says I'm incorrect. I'm not looking for a definite answer since this is my homework but would just like to understand what I am doing wrong.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 25, 2012 #2
    Maybe they're looking for the velocity at the other time the particle is at the origin?
  4. Sep 25, 2012 #3
    I'm under the impression that the origin means they want me to solve it at t=0. What other origin could they be asking for?
  5. Sep 25, 2012 #4
    I guess they mean the origin of the coordinate system, i.e. x=0. You usually say t=0 rather than the slightly ominous "the origin of time".
  6. Sep 25, 2012 #5
    Yeah I just tried now to solve it at x=0 where I find that,
    x(t)= 3t2-7t
    Then I treated it like a quadratic:
    t=7+√[72-4(3)(9)] = 7/3s
    Then I put 7/3s into the formula...and...I got it right, thanks so much.

    Answer ends like so,
    vx=6(7/3)-7= 7m/s
  7. Sep 25, 2012 #6
    There is an easier way to solve it, just factor out one of the t's:
    x=ct^2+bt = t(ct+b),
    for which x=0 when t=0 or t=-b/c. Both solutions are of course valid (based on the problem text), even if the homework website doesn't think so...
  8. Sep 25, 2012 #7
    x(t) = 0 when t=2.3333333333
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook