# What makes the magnitude of the force of friction velocity dependent in one scenario, but not the other?

When a solid objects makes contact with another solid object, I believe the magnitude of the force of friction between them does not depend on the relative speed of the two objects. When a solid and a fluid substance (including gasses) are in contact, however, I believe that the magnitude of the force of friction on the solid does depend on the relative velocity of the two substances.

I'm wondering why this is so. What difference between these two scenarios makes the magnitude of friction force depend on velocity in one, but not the other?

Note: in the original question I talked about " the force of friction". I meant to say the the magnitude of the force of friction though, and have edited it so it says that. This may mean older answers don't make much sense now.

• The situations are completely different, with the physical mechanism of solid-solid friction completely different from the physical mechanism for viscous drag. Incidentally, only in the ideal case is the kinetic friction force independent of the speed difference. For real solids, the coefficient of kinetic friction does vary with relative speed of the solids. Mar 17 '16 at 0:54

I will answer your question by first explaining why fluid friction is velocity-dependent. The resistance between fluid is called viscosity. It is a property arising from collisions between neighboring particles in a fluid that is moving at different velocities. When a fluid is forced through a solid, say a tube, the particles which compose the fluid generally move more quickly near the tube's axis and more slowly near its walls. Therefore, there will be a friction between particle layers, and the friction will be larger if the relative velocity is larger.

For friction between solids(called dry friction) however, because solid particles are bound by electromagnetic force in fixed positions, friction only happens at the surface of contact, and is independent of the relative velocity.

Well, I don't fully understand what your asking, but I don't have a glasses on right now so I might be reading this wrong. To be as simple as possible, if you take your hand (a solid) and your other hand (a solid) and force them together slowly the force or friction is the same as if you put them together faster. With solids speed is not a factor of pressure, friction ect. I'm no genius but I believe your question relates to factors of solubility. If to gases come at each other at a very high speed and neither can dissolve into the other than it might build up pressure. However, if one of them can dissolve or merge with the other than they combined and become one gas.

I hope this helped you (remember I'm not a genius)

Once there is contact between any two objects (whatever phase they may be in,) there will be a friction opposing relative motion between two.In fluid mechanics this is usually called viscosity though.

I believe the magnitude of the force of friction between them does not depend on the relative speed of the two objects. When a solid and a fluid substance (including gases) are in contact, however, I believe that the magnitude of the force of friction on the solid does depend on the relative velocity of the two substances.

I think your contention about the different behavior of solid-solid interface and solid -liquid interface about the velocity dependence of frictional forces do not portray a real picture.

My contention is that in both cases the frictional force is velocity dependent and in case of different nature of surfaces the frictional forces may vary due to smoothness of the surface in contact and relative motion.

The above idea took roots due to microscopic nature of the surfaces in contact and when they are in relative motion the molecular level forces start operating and those interactions are temperature dependent which rises due to relative speed of the surfaces.

Moreover in solids the deformations in the surface terrain when the motion takes place is more pronounced due to the velocity(momentum transfer)

and as velocity rises the frictional force increases but later as the surfaces get smoothed out the friction may decrease in certain cases with velocity. Thus the velocity dependence in solid-solid interface of the kinetic friction is a fact and one should change the classical idea that the kinetic friction is velocity independent (what ever the nature of surfaces)

We show that the friction force varies with the sliding velocity in a manner that depends on the chemical nature of the interface. Surfaces terminated with the hydrogen acceptor and donor moieties capable of forming H-bond networks exhibit a friction that decreases with sliding velocity, a behavior that is opposite (see the recent work in this area.) https://www.researchgate.net/profile/Imma_Ratera/publication/6982094_Velocity_Dependence_of_Friction_and_Hydrogen_Bonding_Effects/links/09e4150f50e92340e6000000.pdf 2.https://hal.archives-ouvertes.fr/ensl-00589509/document