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How hard must you push a box to move it with a constant speed?

  1. Oct 18, 2007 #1
    1. The problem statement, all variables and given/known data
    How hard do you need to push a 2kg box which has .2 coefficient of kinetic friction so that it moves with a constant speed of 5m/s?


    [3. The attempt at a solution
    (I'm calling g 10m/s2 not 9.8)
    F(kinetic friction)=(.2)Fn=(.2)mg=(.2)(20N)=4N
    so...since the acceleration is zero, the force must equal 4N..so 4N is the answer? If I'm correct then in the "5m/s" just a deterrent? Is the key point "constant speed?"
     
  2. jcsd
  3. Oct 18, 2007 #2

    malty

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    Gold Member

    The final acceleration is zero, but how exactly does the block speed up to get to 5m/s?How could you work out the force it needs to accelerate?
     
  4. Oct 18, 2007 #3

    Doc Al

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    Staff: Mentor

    I'd say you were correct. The 5 m/s was just to throw you off!

    The problem could be worded better as it implies you are supposed to figure out how to get it to 5 m/s from some unspecified prior condition. But you aren't given any info as to its initial speed or the time needed to get the final speed, so you can't answer that.
     
  5. Oct 18, 2007 #4

    malty

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    Yeah, that was my initial though too, and tbh I'm a little rusty with this stuff, but what would happen if a force in excess of 4N was applied wouldn't its velocity increase i.e accelerate?
     
  6. Oct 18, 2007 #5

    Doc Al

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    Staff: Mentor

    Sure.
     
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