1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Coefficient of friction for surface

  1. Dec 22, 2008 #1
    a) A block weighing 9.1N requires a force of 2.8N to push it along at constant velocity. What is the coefficient of friction for the surface?
    b) A weight "W" is now place on the the block and 8N is needed to push them both at constant velocity. What is weight "W" on the block?

    This is what I got:
    Since it is constant velocity, there is no acceleration, therefore Fnet is 0, therefore Ff is 2.8N.
    Normal force is (9.1)(9.8) =89.18N
    Formula: Ff=uN

    Therefore u= 0.031
    , but that's not the right answer... Why? what did I do wrong?

    I take it I need to know a before I can do b?
  2. jcsd
  3. Dec 22, 2008 #2


    User Avatar
    Science Advisor
    Gold Member

    Why did you multiply the weight of 9.1 N by 9.8 to get the normal force?
  4. Dec 22, 2008 #3
    Hm? Don't you need to?
    Fg=mg so Fg=(9.1)(9.8), and normal force is going the other direction and Fnet=0, so normal force is also 89.18.

    Isn't that right?
  5. Dec 22, 2008 #4


    User Avatar
    Science Advisor
    Gold Member

    The weight would equal the mass times g, but you weren't given the mass, you were given the weight which is equal to the normal force. Look at your units: N is a unit of force, if you were given mass in kg and multiplied that by g which has units [m/s^2], you would end up with Newtons (again, a force). But what is 1N * 1m/s^2?
  6. Dec 22, 2008 #5
    ohhh. I see what I did wrong, thanks!! You're great help!
    I can't believe my mistake!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?