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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


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    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
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