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Which of these two forces is stronger: The force of static friction or the force of kinetic friction? I am having a hard time understanding this concept of friction, so please explain your answer!

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    $\begingroup$ This will depend very much on the materials/systems you are considering. But in general, it is harder to put an object in motion than to keep it moving. $\endgroup$
    – Demosthene
    Apr 30, 2015 at 15:28
  • $\begingroup$ This is the part I don't understand. Why is it harder to put an object in motion than to keep it moving ? $\endgroup$
    – Al.Ka
    Apr 30, 2015 at 15:53
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    $\begingroup$ See the answer to this question and this hyperphysics page. $\endgroup$
    – Demosthene
    Apr 30, 2015 at 15:59

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Looking very close at the surfaces that touch at friction, this is an illustration

enter image description here

Both surfaces are rough. They have ticks, holes, gabs, pits, spikes, and edges on the microscopic level. The smoother, the lower the coefficient of friction $\mu$. This constant is thus to be considered as a combined "roughness" between these two surfaces.

Intuitively and in general:

Kinetic friction is the situation where the surfaces are moving relative to each other. The surfaces are not being allowed to relax and "sink" into the depth of the gabs. At this point they slide over each other a bit higher than if they "fell" into the holes completely.

For static friction the surfaces relax and spikes fall into nearby pits as deep as possible. From here it is harder to start again, and friction holds tighter.

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When two surfaces are at rest, the only friction between them is static friction. Static friction is a force, and takes a value between 0 and $F_{max} = \mu_\mathrm{s} F_{n}$

When two surfaces are in relative motion, the only friction between them is kinetic friction. Kinetic friction is a force and has a constant value of $F_{k} = \mu_\mathrm{k} F_{n}$

Generally we find that the coefficient of kinetic friction is less than that for static friction for most substances, i.e. once you get an object moving it's easier to keep it moving. However, there is some research into cases where the opposite may be true.

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