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: Differential equation

  1. Apr 15, 2008 #1
    1. The problem statement, all variables and given/known data
    mass m released from a height h falls to the ground under the influence of gravity. It is acted on by air resistance which has the magnitude k times the speed. Show that the mass hits the ground after a time T given by
    T=kh/mg + m/k(1-exp(-kT/m))
    ii)assume two objects of identical size and shape but differing masses are dropped simultaneously. working in first order of k/m determine whether they hit the ground at the same time or if not the time difference.

    2. Relevant equations

    3. The attempt at a solution

    Taking downward direction as positive I have the equation of motion
    m(dv/dt) = mg-kv
    (dv/dt) = g-(k/m)v

    integrating from 0 to v(t) and from 0 to T using separable equations I got
    (m/k)ln(1+m/(kv(t))) = T
    v(t) = (1/k) exp (-kT/m)(m+1)

    Could someone tell me if this is right and how to go about finding the time of flight?

  2. jcsd
  3. Apr 15, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    You are leaving out too many steps. Those aren't right and you omitted the details of the steps where you are going wrong. For example, whatever happened to 'g'? In a correct answer as t->infinity, v*k should approach m*g, right?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook