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Homework Help: Help on friction and motion

  1. Oct 2, 2004 #1
    Hi everybody. :smile:

    I'm not good at physics/math but I really want to (someday...) and this problem for a lab next week has been bugging me for the last... 5 minutes. It says:

    For a block moving at constant speed, if the applied force just balances the frictional force, why does the block move?

    I have no idea... And yes I'm stupid if that's what you're thinking. >_<

    If it moves at constant speed then there's no acceleration.. and I'm thinking total forces balance out to 0... so why does it move? :cry:
  2. jcsd
  3. Oct 2, 2004 #2
    [itex]\sum \vec F=m\vec a[/itex]

    In this case: [itex]\sum \vec F=m\vec a=0[/itex]

    The particle has mass, so the acceleration is zero. You're given an initial condition where the block is in motion. No acceleration--no speeding up or slowing down.
  4. Oct 2, 2004 #3
    sum of all external forces = mass x acceleration,
    so if sum of external forces = 0, then acceleration must be 0
    acceleration = 0 doesn't mean it will stop..
    but acceleration < 0 (i.e. decceleration) will slow it down until it stops
    in Newton's first law:
    An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
    in this case, there's no unbalanced force.. therefore your block will move with constant speed
    actually if there's no applied force that is balancing the frictional force, the acceleration become decceleration (a < 0) and then your block will stop moving..
  5. Oct 2, 2004 #4


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    Newton's 1st Law

    [tex] \sum_{i=1}^{n} \vec{F}_{i} = 0 \rightarrow \vec{V} = constant [/tex]
  6. Oct 2, 2004 #5
    Ok I know that the forces equal to zero... but it's still moving. The only way I can see the answer as to why it's moving is because of newton's first law... but that means my answer should be something like:the block's ORIGINAL state was that it was moving and as long as the forces total to 0 then it will continue to be in that state until an unbalanced force acts on it...

    Somehow I feel that still doesn't answer why it's still moving. I mean when I push it and accelerate my force has to be greater then friction but when I continue to push it in CONSTANT velocity the forces balance out to 0 and it's in equilibrium state while moving...

    I don't know I think I'll just write that. Thanks for the help. ^_^
  7. Oct 2, 2004 #6


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    If there was no friction, you could push a block, and it will continue to move at constant velocity, what stops objects in real life are forces such as friction.
  8. Oct 2, 2004 #7
    Where in your description of the problem does it say you are pushing the box and accelerating it? You presented an initial condition--the box is moving. You are told a pushing force equals the frictional force. That's it. If you want to see this problem in action you can push a box at constante speed across a floor. Whne you do the force you apply equals the force of friction. If you push a box a constant speed does it speed up or slow down? No--the forces sum to zero thus acceleration is zero thus the box will move at the constant speed.

    This is correct BTW.
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