# Why is kinetic friction less than static friction? [duplicate]

Ex: A wooden block is lying on a table.

I am told that because the block is still, the microscopic surface irregularities form more complicated interlocking structures. Is it because the force of the block on the table deforms the molecular structure of the table and wood to eventually reach an equilibrium state? This state then is more connected and harder to change than a block simply gliding over the table? Also, how fast does this happen?

• Related: physics.stackexchange.com/q/541/2451 , physics.stackexchange.com/q/16633/2451 and links therein. Commented Oct 27, 2015 at 19:08
• Note there are several cases where the coefficient of kinetic friction is (nearly) the same as that of static friction (Delrin on Delrin is a classic example). Commented Oct 27, 2015 at 19:59

A body in motion tends to stay in motion.

Suppose the peaks of one surface aligned with the valleys of another surface, and you applied enough force to start slipping. One of two things must happen. Either the peaks get shaved off (which takes a lot of force), or the average distance between the two objects increases. Even if there is a little shaving taking place, the distance between the objects is increasing. If the objects start separating, they cannot suddenly return to have the peaks and valleys line up. It takes time, just like it takes time for a ball thrown into the air to return to earth. If the surfaces are kept in motion, the distance between them will reach some average that is greater than the average when they are at rest. Greater separation means that only the peaks of the surfaces are coming into contact at points closer to the tips of the peaks. There will be a reduced attraction between the molecules of the two objects, and a reduced component of the microscopic normal forces parallel to the direction of motion. This results in a lower component of force parallel to the direction of motion, i.e., less friction.

Consider the logic if the reverse were true: kinetic friction was greater than static friction.

Then you could have the situation where static friction is too small to stop an object on a ramp from sliding, but kinetic friction is great enough to stop the same object when it was moving.

So what is an object to do?

• Just because static friction is smaller than kinetic friction doesn't mean that it can't prevent an object from moving. What would make it move? There is no force of kinetic friction at v=0 and as long as the sum of all other forces is smaller than the friction force, nothing moves, at all. Commented Oct 27, 2015 at 23:50
• @CuriousOne: What if the object is on an inclined plane? Or under the influence of any force at all? Commented Oct 28, 2015 at 0:14
• I assumed the standard physics lab set up of a block on a ramp. Edited... Commented Oct 28, 2015 at 0:16
• @Javier: If the object is on an inclined plane then it will start to move as soon as the force component parallel to the plane exceeds the friction, no matter what the coefficient of friction is at that point. As a general remark, the high school theory of friction is mostly oversimplifying mythology. Friction is a very complicated mesoscopic phenomenon for which no closed theory exists. In practice the actual friction force depends on minutiae down to the level of molecular layers on surfaces, which are extremely hard to control in experiments. On the upside... that's why oil and grease work! Commented Oct 28, 2015 at 0:21