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Magnitude of object with constant speed

  1. Feb 24, 2012 #1
    1. The problem statement, all variables and given/known data
    in the physics laboratory experiment, a 6 kg box is pushed across a flat table by horizontal force F. if the box is moving at a constant speed of 0.35 m/s and the coefficient of kinetic friction is 0.12. what is the magnitude of F.

    2. Relevant equations
    F = friction
    μ = coefficient of friction
    Fn = normal force

    Fn = mass of object * acceleration

    3. The attempt at a solution

    i solve for Fn first,
    Fn = 6 kg * 0 m/s2
    and got an answer of zero for normal force.... i let the acceleration to be zero because it says that it has constant speed..... Is it normal to have a zero normal force?

    and then when i solve for the magnitude of friction....
    F = 0.12*0
    which is zero also... o_O.....

    i'm not sure if i got a wrong equation, but i think its correct...... or if i got some wrong interpretation of the given values that results to zero friction.......

    My question is, is it a normal thing for an object with constant speed to have a zero friction?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 24, 2012 #2

    Doc Al

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    Staff: Mentor

    That's not how you find the normal force. Consider the vertical forces acting on the box.
  4. Feb 24, 2012 #3
    thanks for reply...

    so ill gonna consider the acceleration due to gravity of an object with no horizontal acceleration?
  5. Feb 24, 2012 #4
    First and fore most, never solve Newton Laws Problems without FBD. Draw the figure and correctly denote the directions in which the forces acts. Use F(net)=Ma along x-y axes. See what happens.
  6. Feb 24, 2012 #5

    Doc Al

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    Staff: Mentor

    Sure. You'll need it to find the object's weight.

    Just because you might use the acceleration due to gravity does not mean the object must be accelerating. Think of the acceleration due to gravity as a measure of the strength of the earth's gravitational field.
  7. Feb 24, 2012 #6
    In this case
    ##F_N=mg## m = mass, g=gravity, gravity pulls down with 9.81m/s^2 so find normal force
    If you're trying to move the body in constant velocity then
    ##F_{net}=0## because:
    ##F-F_f=m * 0##
    ##F-F_f=0## so friction is equal to pushing force.
    so simply find friction ##F_f=\mu F_N## and that's the pushing force
  8. Feb 25, 2012 #7
    thanks !!! its clearer now... :)
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