1. Limited time only! Sign up for a free 30min personal 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!

SHM with friction

  1. Nov 2, 2007 #1
    Suppose a mass is oscillating attached to a spring in a horizontal surface with friction. Friction sooner or later will stop the oscillation. Does exists any possibility that the oscillating object stops at equilibrium position of that SHM, or it will stop elsewhere but never in the equilibrium position, i.e., x=0? TIA.
  2. jcsd
  3. Nov 2, 2007 #2
    It's certain to stop at the equilibrium position. The reason SHM happens is because there's a restoring force that acts to return the particle to that position; as the system loses energy it can no longer store energy in the potential creating that force (e.g. gravity in the case of the simple pendulum), so the amplitude of oscillation dies off.
  4. Nov 2, 2007 #3

    Meir Achuz

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It need not stop at x=0 if there is static friction.
    It will stop when the retarding kinetic friction force finally brings v to zero.
    This could occur at x=0, as well as other x.
  5. Nov 3, 2007 #4
    The retarding kinetic friction force is constant, so as the oscillator slows down, there will be a moment that the restoring force has a shorter value than kinetic friction force, and this never occur at origin, so i suppose it cant never stops at x=0.
  6. Nov 3, 2007 #5
    If the frictional force exceeded the restoring force, the "oscillator" would never oscillate; when displaced by a small distance the frictional force would exceed the restoring force, and energy would stay stored in the stretched spring.
    The force experienced by the particle isn't affected by its velocity; only by the spring constant and the instantaneous displacement; F=-kx.
  7. Nov 3, 2007 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    Not necessarily. The position where it stops will depend on where it starts. If the damping force is constant then there are two positions from which the oscillator is started and will stop at the origin.
  8. Nov 3, 2007 #7
    Thanks to all who contribute to clarifying this question!

    What do you mean when you wrote this:
    "If the damping force is constant then there are two positions from which the oscillator is started"

  9. Nov 3, 2007 #8


    User Avatar
    Staff Emeritus
    Science Advisor

    There are two positions from equilibrium. For example, take a spring. The force could be tension or compression with a displace that is + or - from the equilibrium (zero displacement) point. In other words, there is a symmetry in SHM.
  10. Nov 3, 2007 #9
    For me is difficult to understand why it can reach x=0 with friction.
    Let´s explain if i understood:
    When oscillating with friction there will be a moment where [tex]\mu[/tex]N=kx. and after that [tex]\mu[/tex]N>kx, so for the same spring the heavier the object the longer from the origin it will stop, unless when [tex]\mu[/tex]N=kx it has enough kinetic energy to dissipate and stop elsewhere, that can include point x=0! Is this interpretation correct?
  11. Nov 3, 2007 #10


    User Avatar
    Staff Emeritus
    Science Advisor

  12. Nov 4, 2007 #11
    If this bit was right, there'd simply be no oscillation. That SHM has already started to occur is indicative that there is no such moment.

    Remember that in SHM the energy contained by the system doesn't directly affect the force experienced by the particle. The particle doesn't stop because of frictional forces balancing the restoring force of the spring. That can happen, but then you don't get SHM.

    What happens instead is that work is done against friction and energy is lost from the system. As the system loses energy, the mass has less energy to store as potential energy in the spring, so the amplitude of the oscillation dies off to zero.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: SHM with friction
  1. SHM Problem (Replies: 1)