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!

Displacement (Answer Key Error)

  1. Nov 6, 2013 #1

    Qube

    User Avatar
    Gold Member

    1. The problem statement, all variables and given/known data

    The position of an object traveling in a straight line at time t is given by s(t) = (2/3)t^2 + 4t^2 + 6t + 2.

    What's the displacement after 3 seconds?

    2. Relevant equations

    Displacement is final position minus initial position.

    3. The attempt at a solution

    Displacement at 3 seconds is s(3) - s(0).

    s(0) = 2. Note the constant term!

    s(3) = 18 - 36 + 18 + 2.

    s(3) - s(0) = 0.

    Interestingly enough, my key claims displacement is simply s(3).

    I'm guessing this is an error (and not on my part)!
     
    Last edited: Nov 6, 2013
  2. jcsd
  3. Nov 6, 2013 #2
    Can you explain how 2/3 * t^2 = 4*t^2 + 6*t + 2?
     
  4. Nov 6, 2013 #3

    Qube

    User Avatar
    Gold Member

    Sorry, replace the second equal sign with a plus sign. I've corrected it in the OP.
     
    Last edited: Nov 6, 2013
  5. Nov 6, 2013 #4
    OK, can you also show how you're getting s(3)? Because, if the equation is s(t) = (2/3)t^2 + 4t^2 + 6t + 2, then at t = 3, the current position is 6 + 36 + 18 + 2. Which is equal to 62. If the equation is s(t) = (2/3)t^2 - 4t^2 + 6t + 2, then at t = 3, the current position is 6 - 36 + 18 + 2, which is equal to -10. Not sure if you're copying the equation wrong or what...
     
    Last edited: Nov 7, 2013
  6. Nov 6, 2013 #5

    Qube

    User Avatar
    Gold Member

    I think I did copy the problem wrong, but I know there was a constant at the end, which makes the answer key unequivocally wrong, correct?
     
  7. Nov 7, 2013 #6

    Qube

    User Avatar
    Gold Member

    Okay, I found it!

    http://i.minus.com/jW4oBtV2mWi3O.png [Broken]

    http://i.minus.com/jbrSQnX1kxCiGG.png [Broken]
     
    Last edited by a moderator: May 6, 2017
  8. Nov 7, 2013 #7
    Well, it definitely seems like now there is an error in the answer key. So, yes, I would not follow it.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted