Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Kinetic Friction not so easy

  1. Sep 20, 2007 #1
    1. The problem statement, all variables and given/known data
    The block is pushed to the right at a constant velocity. If the coefficient of kinetic friction is 0.5, what is the magnitude of the pushing force?


    2. Relevant equations
    I dont know...


    3. The attempt at a solution
     
  2. jcsd
  3. Sep 20, 2007 #2

    learningphysics

    User Avatar
    Homework Helper

    Did they give the mass of the block? What's your first idea? Give it a shot...
     
  4. Sep 20, 2007 #3
    no they didnt give the mass of the block, that is why im so confused my teacher said something about sigma Fx=0 and sigma Fy=0 but i dont know what to do with those numbers, all i know is velocity is constant which makes acceleration 0 which makes force of x and y 0, but after that im lost
     
  5. Sep 20, 2007 #4

    learningphysics

    User Avatar
    Homework Helper

    Yes, net force in the x direction is 0. Net force in the y-direction is 0.

    What are the forces acting in the y-direction?

    What are the forces acting in the x-direction?
     
  6. Sep 20, 2007 #5
    In the x direction is the pushing force and force of friction and in the y direction is force of gravity on a box whos mass is not given and the normal force, i think the lack of given numbers is confuing me
     
  7. Sep 20, 2007 #6

    learningphysics

    User Avatar
    Homework Helper

    Yeah, just use the variables... let mass = m.

    Write the [tex]\Sigma\vec{F} = ma[/tex] equation for the x-direction... then for the y-direction... just use the variables...
     
  8. Sep 20, 2007 #7
    in x direction it would be (mass)(0) so resultant force would be 0, because acceleration would be 0 due to constant velocity and in y direction (mass)(9.8) and then i dont really know what you could do with that...
     
  9. Sep 20, 2007 #8

    learningphysics

    User Avatar
    Homework Helper

    Write the equations using Fpushing, friction, mg, Fnormal... how do they add or subtract?
     
  10. Sep 20, 2007 #9
    mg-Fnormal=0

    this is where i get confused with x because they are not in equilibrium so they wouldnt equal 0
    Fpushing-friction=?? idk
     
  11. Sep 20, 2007 #10

    learningphysics

    User Avatar
    Homework Helper

    It equals 0, because acceleration is 0 (constant velocity). Now, you also know that friction = [tex]\mu * F_{normal}[/tex]

    Solve for Fpushing using your two equations, in terms of mass, [tex]\mu[/tex] and g.
     
  12. Sep 20, 2007 #11
    ok, i understand why it equals zero now but when you use that equation would it be
    friction=(0.5)(9.8m) and which two equations are you referring to?
     
  13. Sep 20, 2007 #12

    learningphysics

    User Avatar
    Homework Helper

    Yes, that's right...

    The two equations I meant were:

    Fpushing - friction = 0
    Fnormal - mg = 0

    you actually used the second equation to get friction = 0.5*9.8*m

    So what does Fpushing come out to...
     
  14. Sep 20, 2007 #13
    Fpushing-(0.5)(9.8m)=0
    Fpushing-4.9m=0
    Fpushing=4.9m??
     
  15. Sep 20, 2007 #14
    Seems good to me.
     
  16. Sep 20, 2007 #15

    learningphysics

    User Avatar
    Homework Helper

    yup. looks good to me too.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook