Physics Formula For Force

Friction, or the force of friction, is used to analyze and determine potential movement between the surfaces of two contacting bodies. More specifically, it is the force that resists movement, and consists of several different types. The kind that people are probably most familiar with is called dry friction - as opposed to fluid friction, skin friction, and internal friction. Dry friction is divided into static friction and kinetic friction.

Static Friction and Kinetic Friction

The name pretty much says it all, but here's a basic rundown for good measure. Static friction is the frictional force that resists movement between stationary bodies, or bodies that are not moving relative to each other. The force of friction between a rubber tire and pavement that enables a vehicle to accelerate is a good example of static friction.

This can be mistaken for kinetic friction due to the fact that both the tire and vehicle are moving. However, because the given section of rubber that is in contact with the pavement is stationary relative to the pavement, it is static friction. If it wasn't for the force of static friction, the vehicle would not be able to accelerate - or at least not very fast.

A good example of kinetic friction is when you slam on the brakes while traveling at a high speed, locking the wheels so as to skid on the pavement. From the point the rubber tire begins skidding to the point you either release the brakes or come to a complete stop, it will experience kinetic friction.

The Coefficient of Friction COF and Traction

Traction is another way of describing the maximum value of static friction prior to movement or slippage. But in order to determine the value of traction, the coefficient of static friction (COF) between the two bodies in question becomes needed. The COF is a value that cannot be derived by calculations but must be found empirically through experimentation.

But thanks to smart people with the time and know-how needed, these values can for the most part be found in various publications, handbooks, and resources, available even for free on the internet. The reason it can't really be calculated via specific formulas is because every substance is slightly different, and various factors such as temperature and atmosphere often affect accuracy.

The coefficient of friction is the ratio of the force of friction between two bodies and the normal force, or the force that is pressing them together - equal to the weight-force of the upper of the two bodies. There are two types of coefficients of friction: the coefficient of static friction and the coefficient of kinetic friction. The coefficient of static friction is usually greater than that of kinetic friction.

The COF of two solid bodies can also vary significantly depending on whether it is dry friction or what is called lubricated friction - a form of fluid friction. This may seem obvious, but there are also certain materials whose COF remains the same regardless of the presence or absence of lubricant such as steel on teflon and steel on polythene.