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Homework Help: 'Rocket equation'(Physics by example, Rees, pr. 16)

  1. Feb 8, 2012 #1
    attachment.php?attachmentid=43673&stc=1&d=1328744018.jpg

    Questions:
    1. How can one tell that at time t+dt the velocity of the rocket is an infinitesimal quantity and the velocity of the ejected product a real quantity?
    2. Why is for the mass of the rocket not simply written m-dm and why for the mass of the ejected product not written (+)dm?
     

    Attached Files:

  2. jcsd
  3. Feb 8, 2012 #2
    The rocket starts from rest at time = t. If dt is small enough then the rocket has gained little velocity at time t + dt.

    I'm not sure what you mean when you say,

    "...the velocity of the ejected product a real quantity?"

    You write,

    "Why is for the mass of the rocket not simply written m-dm and why for the mass of the ejected product not written (+)dm?"

    That would have made more sense to me as well, but it still works out.
     
  4. Feb 10, 2012 #3
    Hi Spinnor,
    In the solution, the velocity of the product is denoted with vr. So they are saying that the velocity is a real quantity (consists of a real number) as opposed to an infinitesimal quantity (like dv, a hyperreal number).

    Ok, I think I understand this now.
     
    Last edited: Feb 10, 2012
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