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!

Math problem regarding kinetic motion

  1. Sep 19, 2007 #1
    Sorry, i have problem in this question which comes from my textbook. I couldn't solve it and i would like someone to guide me.

    A body travels 30 metres in the sixth second of motion and 24 meters in the ninth second. Find:
    i) the initial velocity (ans: 41 m/s )
    ii) the retardation, assumed uniform (ans: -2 m/s/s )
    iii) the time elapsing before coming to rest (ans: 20.5s)

    From my thinking is that, the body is actually moving in a curve so it changes its displacement from 30 meters to 24 meters within the time [6~9] seconds,
    but this problem is in charge of kinetic problem as well, as it's in the chapter 'kinetic'.
    And since it's not moving in a straightline, i assume that it cannot apply the ordinary equation : S = u * t * 1/2 a*t^2;
    But by only reconstructing the equation from a = dv/dt;

    Plus, the retardation and initial velocity seems to be dependantly constant variable. So my guess is that, since it involves friction, i would simply guess that it's actually having altering of work as :
    Sum(W) = Fn * d(Sn);
    where W is the total Work,
    Fn is the changing of Forces within the time interval [0,t];
    dSn is the delta of distance traveled within time interval [0,t];

    Thanks in advance.
  2. jcsd
  3. Sep 19, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    From my thinking, this means that the velocity changes from 30 m/s to 24 m/s in the [6, 9] s time interval.

    Also, "the retardation is assumed uniform" sounds to me like "the acceleration is constant", which means the formula
    [tex]s(t) = s_0 + v_0 t + \tfrac12 a t^2[/tex]
    will certainly hold (it follows from solving
    [tex]s''(t) = a[/tex]
    with the appropriate boundary conditions).

    "Kinetic" basically just means "movement". E.g. "kinetic energy" is just "movement energy", in a quite literal translation.
    But perhaps what is meant is, that you should solve it using energies instead of dynamic equations?
  4. Sep 19, 2007 #3


    User Avatar
    Science Advisor

    Why do you assert "it's not moving in a straightline"? I can find no where in the problem statement that it is said it is not moving in a straight line. "Kinetic" simply means "moving"- not necessarily moving in a curve.

    Assuming constant acceleration, a, (although that's not mentioned until (b)), a body will move (1/2)at2+ v0t where v0 is the initial speed. After 5 seconds, it will have gone (25/2)a+ 5v0. After 6 seconds, it will have gone (36/2)a+ 6v. During the "6th second", it will have gone (36/2)a+ 6v0- ((25/2)a+ 5v0= (11/2)a+ v0= 30. Do the same for the distances traveled in 8 and 9 minutes, setting the difference equal to 24 and you have two equations to solve for a and v0.
  5. Sep 20, 2007 #4
    oh yes! i misunderstood the question by assuming it has "travelled along". Thanks everyone for the answer and detailed explanation. I got it solved. ^^
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook