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: Forming a differential equation for a body whose mass changes

  1. Feb 19, 2009 #1
    http://img8.imageshack.us/img8/2166/58510589xd3.th.jpg [Broken]

    I was having trouble following various aspects of the above explanation:

    Why does the ejected mass have a speed "v-u" at the start of the interval?

    Why is the oif the body mass (m + (Delta)m) and not (m - (Delta)m).

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 19, 2009 #2


    User Avatar
    Gold Member

    The speed has to be v-u since the mass is being ejected relative to the body it is being ejected from. To an outside observer it would be v+u since the mas and the body are apparently going in the same direction.

    "Why is the oif the body mass (m + (Delta)m) and not (m - (Delta)m)."

    As the body moves the mass changes [tex]\delta m[/tex] doesn't necessarily means mass is added...simply means a change in mass at another time t
    Last edited: Feb 19, 2009
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook