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: Maximum possible static friction

  1. Oct 28, 2005 #1

    Is maximum possible static friction between a particular body and a particular surface equal or smaller to k(s) * N ?

    k(s) means coefficient of static friction

    If it is smaller, then only when force pulling on object is greater than C(s)*N will body start moving ?

    I'm confused since I found few articles on the net saying max_possible_stat_fric < k(s) * N, while others even saying max_possible_stat_fric equals k(s) * N and yet at the same time claiming when both the pulling force and max_possible_stat_fric are equal to k(s) * N the object starts moving, which doesn't make sense since the two forces would cancel each other out

  2. jcsd
  3. Oct 28, 2005 #2

    Chi Meson

    User Avatar
    Science Advisor
    Homework Helper

    The force of static friction is less than or equal to (coefficient of s. friction)x(normal force). The maximim possible is equal to "k(s)*N" (as you put it.

    The reason why static friction can be less, is due to the fact that if nothing is pushing on a stationary object, there is no frictional force at all on it. If a small applied force is put on the object, and the object does not move, then the static friction only has to be enough to balance the small applied force.

    If an applied force that exactly equals the max static friction is on an object, technically forces are balanced and the object does not move; but if this applied force is infinitessimally larger, then the object will accelerate. Let's say the maximum static frictional force is 25 N. An object will begin to accelerate if you apply a force of 25.0000000001 N. This is why they say that the "minimum force required to start the motion is equal to the maximum static frictional force." Rounding to significant digits, 25.000000000001 equals 25.
    Last edited: Oct 28, 2005
  4. Oct 30, 2005 #3
    thank you. It's strange tho that they wouldn't mention that since it can confuse a person
  5. Oct 30, 2005 #4

    Chi Meson

    User Avatar
    Science Advisor
    Homework Helper

    "They" do this all the time.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook