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: TA need help grading HW question!

  1. Sep 27, 2010 #1
    I'm a TA and there is an inconsistency in what the solutions manual states and what the students are turning in, but both seem correct.

    The Latex code isn't working properly for the post, but it is easy to see what equations are used

    1. The problem statement, all variables and given/known data
    A particle of mass m moves along the x axis. Its position varies with time according to [tex]$x$=2t^3 - 4t^2[/tex] , where x is in meters and t is in seconds. Find

    a. The velocity and acceleration of the particle as functions of t.

    b. The power delivered to the particle as a function of t

    c. the work done by the net force from [tex]t=0 $to$ t=t_{1}[/tex]

    2. Relevant equations
    The first 2 parts are easy and the answers are given by
    [tex]$v$=6t^2 - 8t $;$ $a$=12t - 8 $;$ $P$=power=8mt(9t^2 - 18t + 8)[/tex]

    3. The attempt at a solution

    The problem is that both solutions are correct mathematically, but they are different. Therefor one must be wrong and I need someone to tell me which on, and WHY?

    Solution manual
    [tex]$w$=\int_0^t \! P \, dt
    $w$=\int_0^t \! 8m(9t^3 - 18t^2 + 8t) \, dt
    Student Solution
    [tex]$w$=F\Delta x=ma\Delta x = m(12t - 8)(2t^3 - 4t^2)[/tex]
    This becomes,
    [tex]$w$=m(24t^4 - 64t^3 + 32t)[/tex]
    Which one of these is wrong, since the math is right on both of them and the equations are valid for power?
  2. jcsd
  3. Sep 27, 2010 #2
    The second equation (Work = force*distance moved) is only valid for constant forces, in fact you can derive it from the first equation by letting P = F*dx/dt for some constant F.
  4. Sep 27, 2010 #3
    So the student solution is incorrect?
  5. Sep 27, 2010 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Yes, it is incorrect. The correct equation is
    W = F dx
    Which, as marmoset said, equals F Δx only for a constant force.
  6. Sep 27, 2010 #5
    students looks wrong to me
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook