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Homework Help: Easy one here yet, i can't get it

  1. Nov 6, 2007 #1
    A 1200kg space probe travelling initially at a speed of 9500 m/s through deep space fires its engines that produce a force of magnitude 92000N over 86 km distance. Determine the final speed of the probe.

    Attempt at solution:
    I tried finding the acceleration by dividing the mass by the force. Then i tried subbing into the equation v^2=v^2+2ad

    Is it correct because i get the same value as the original velocity?
  2. jcsd
  3. Nov 6, 2007 #2
    It sounds correct. You have a constant force, and thus a constant acceleration. Using the kinematic equation in your post, you should be able to find the final velocity. Don't forget to convert the km to meters.

    Edit: By the way, you shouldn't get the same value as your initial velocity. A constant acceleration is taking place, and therefore your velocity is going to change.
  4. Nov 6, 2007 #3


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    You seem to have the correct method assuming that you don't need to take into account any mass lost. I would presume there is just a calculation error somewhere. If you post exactly what you're doing to get the wrong answer then perhaps someone can see where you made the mistake.
  5. Nov 6, 2007 #4
    Have you converted the 86km into metres? It is impossible for v = u, as hotcommodity said, acceleration is occurring. I calculated, and v does not equal u. :smile:
  6. Nov 6, 2007 #5
    Hot commity ,, u are a genius, it seemed that i wasn't converting the distance from kilometers to meters. It was right in front of me all along, Thanks for the help all, i really appreciate it ... another thing thats bothering me is this question:

    A student a compressed spring of force constant 22 N/m to shoot a 7.5*10^-3 kg eraser across a desk. The magnitude of the force of friction on the eraser is 4.2*10^-2. How far along the horizontal desk will the eraser slide if the spring is initially compressed 3.5cm?

    Attempt ---> im thinking something along the line of Fd=1/2kx^2 but i seem to get stuck

    any ideas
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