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Homework Help: Force of declerating object losing mass

  1. Feb 28, 2015 #1
    1. The problem statement, all variables and given/known data

    A spacecraft travelling in free space, at a speed of 6 m s -1 has a mass of 10,100kg (including fuel)

    A rocket engine on the spacecraft is fired up for 4 seconds, ejecting 100kg of fuel and slowing the spacecraft down to 0.02 m s -1

    What is the force in newtons, feing the 4 second burn of the spacecraft.

    2. Relevant equations

    3. The attempt at a solution
    I really new to physics, and this is an example question in order to help us understand how to work these type
    Of equations out.

    I know that f = m a, however in this case I think the fact that the spacecraft is losing weight needs to be taken into acount.

    The momentum before is 60,600 kg -1 s -1

    As I said before Im really new to physics and being able to understand the reasoning for the formula is my main aim here.

    Thank you in advance to any help that may be given here.
  2. jcsd
  3. Feb 28, 2015 #2


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    Staff: Mentor

    Hi Lukec29. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    It's probably good enough to use the average of its mass during that time in your calculations.

    Do you know what the expected answer is?
    Last edited by a moderator: May 7, 2017
  4. Feb 28, 2015 #3
    Unfortunately not, my tutor is on holiday currently. Im more looking for the correct workings so I can understand the formula and put it to practice.
  5. Feb 28, 2015 #4


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    Science Advisor

    As long as the mass is decreasing "linearly", at a constant rate, then you can use the average mass as if it were the mass the entire time.
    Initially, the mass, including fuel is 10,100 kg. At the end of the 4 seconds firing, the mass is 100kg less (I am assuming you did NOT mean "100 kg per second") so the average mass, over the 4 seconds is [10,100+ (10,100- 100)]/2 kg. Use that with "F= ma".
    Last edited by a moderator: Feb 28, 2015
  6. Feb 28, 2015 #5


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    Science Advisor
    Homework Helper
    Gold Member

    What do you know about conservation of momentum?
  7. Feb 28, 2015 #6
    Nothing at all, it's degree level but first year, very simple.

    Would force in Newtons, from the average mass of before and after ejecting the fuel suffice
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