Rolling Friction vs Static Friction

[Arkandea]

Hi everyone I am new here =)

Can someone please explain whats the difference between the two types of friction I have mentioned? I am aware rolling friction is force that opposes direction of movement but static friction can either be in the same direction of movement or resisting it? I'm talking about wheels of course.

Thanks

[QuantumJG]

There are three different kinds of friction:

- Static friction (f_s). Say we are pushing an object with force, F_push, then:

  f_s F_push.

  f_s(max) = μ_sN 

  This basically means that if:

  - F_push > μ_sN, the object will move.

  - F_push = μ_sN, the object will be on the verge of moving.

   - F_push < μ_sN, the object won't move.

- Kinetic Friction (f_k). The kinetic friction force is nearly always constant, so:

  f_k = μ_kN

- The rolling friction force (f_k). The rolling friction force is also nearly constant so:

f_r = μ_rN

Another fact:

- μ_s > μ_k > μ_r

The reason being for this is that friction is due to the molecular bonds an object makes.   




 

[appianway]

Adding on from Quantum, the coefficient of static friction is usually larger (but not always) than the coefficient of kinetic friction because, quite simply, the particles have much less time to form the bonds.

When considering the direction of the frictional force, consider the direction that motion is likely to be in if motion were to occur (ie if the frictional force was to be lessened). It can't really be said friction can be in the same direction of motion - if it's static, it's not moving. In some situations, the direction of motion could occur in either way - in these instances, complete two models looking at the frictional force in both directions (this might come into play in circular motion if you have something moving with the plane that it's lying on).

The coefficients form nice approximations, but just keep in mind that they're approximations.