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Confusion on mechanics.

  1. Sep 16, 2012 #1
    Hello, I just started basic mechanics and what seems to mess with me is this:

    take a 1kg cube in a string, with how much force do you need to push it to give it the even speed of 2M/S, In the book it says F1-F2=0, so 9,8N=F2, so you have to push it upwards with 9,8 newtons... but how the hell does that mean it get's speed... why does it not stop 100% since it has equal force on both sides, and even more, why does it get 2m/s and not 3 or 4m/s... this is what has been messing with me, I can't seem to get it.
     
  2. jcsd
  3. Sep 16, 2012 #2

    mfb

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    Staff: Mentor

    There is no relation between force and speed as you want to get.
    With a force of 9.81N upwards from the string, the net force on the cube is 0, and it will keep its current velocity (can be 0, but it can be anything else, too).
    If the force is different, it will accelerate - its speed will always increase, until the forces change. At some point, it reaches 2m/s, but that value is not special in any way.

    As example, if the string applies a force of 10.81N upwards, the net force on the cube is F=1N upwards, and it accelerates with a=F/(1kg)=1m/s^2 upwards. If it is at rest initially, it will have a speed of a*t=1m/s after one second and 2m/s after two seconds.
     
  4. Sep 16, 2012 #3
    If the question is worded exactly as you wrote it, I would say it is a poorly worded question. It should say something along the lines of "what force do you need to apply to maintain the velocity of 2m/s". You are correct that 9.8N is the force needed to maintain any constant velocity (excluding all of the annoying things like friction). If you were going to accelerate from 0m/s to 2m/s, for a period of time you would need more than 9.8N until it reached that velocity.
     
  5. Sep 16, 2012 #4

    Thanks alot, i get it much better now... the question was just silly because they instead of saying equal speed they said specifically 2m/s equal speed.
     
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