Enter the normal force and friction force into the coefficient of friction calculator below. Evaluate the coefficient of friction between two surfaces.
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Coefficient of Friction Formula
The following formula is used in the coefficient of friction calculator above:
u = F / N
- Where u is the coefficient of friction
- F is the force due to friction
- N is the normal force
This can be used in both static and kinetic friction cases. In a static friction case, the force applied, P, will never exceed the frictional force F. Otherwise the object would be in motion and this would be a kinetic friction case.
Coefficient of Friction Definition
A coefficient of friction is defined as the unitless factor between two surfaces that resist movement when under a normal force.
Are you taking an AP physics class or you’re taking an introductory physics class in college? Then, you might have stumbled on this term. So, what’s the coefficient of friction?
The value of each coefficient of friction depends on the materials of the two bodies concerned. The lower the value, the easier it is for the object to move (considering that mass is constant), and vice versa.
The coefficient of friction, symbolized μ, is a scalar value representing the amount of resistance two bodies exert on each other. You can calculate the coefficient of friction using this formula:
μ = f÷N, where:
- μ is the coefficient of friction
- f is the frictional force
- N is the normal force
There are two types of coefficient of friction (COF): static and kinetic. Generally, the kinetic coefficient of friction between two bodies is lower than the static coefficient.
For example, you’re trying to push an object and it’s not moving. This is the coefficient of static friction. However, when you push hard that the object finally starts moving, this is called maximum force.
On the other hand, when an object moves, it has what’s known as ‘frictional force.’ This is where kinetic COF comes in.
The definition of a good COF depends on what you’re trying to do.
Say you’re trying to determine a good COF for a rubber variant you’re using to make car tires. In this case, you’d need a variant with a high enough COF to ensure stability on the road. At the same time, it shouldn’t be too high that it hinders movement.
However, if you’re trying to make gears out of new material, you want them to have a relatively low COF. This way, you can eliminate friction and reduce erosion.
Materials react differently with lubrication. Yet, the general rule is that the drier the material, the lower the COF.
The general range for lubricated and dry materials is 0 to 1. Yet, most materials in dry conditions have COF that range between 0.3 and 0.6. There are instances where certain values go outside this range, but it’s pretty rare.
The highest known COF is 1.2 between rubber and solids. Sometimes it’s a tie between silver and silver. However, the highest known coefficient of kinetic friction is between aluminum and aluminum, which is measured at about 1.4.
One of the lowest known COF is between the synovial joint fluid and human cartilage. It’s set at about .01.
Even lower is PTFE, or Teflon, which is measured at 0.04. Likewise, a mix of titanium diboride and aluminum-magnesium boride also has a low COF ranging between 0.04 and 0.05.
Friction is an inevitable part of the forces acting on a moving body, not just in classical physics but also in real life. That’s why it’s worth reading more about it and discovering all its wonders that have been around since the dawn of time.
How to calculate a coefficient of friction?
How to calculate coefficient of friction
- First, determine the net horizontal force acting on an object.
Calculate the net force acting on a stationary or moving object.
- Next, determine the normal force acting on an object.
When analyzing a single object with no extra vertical forces, the normal force should be equal to the weight.
- Calculate the coefficient of friction
Calculate the coefficient of friction using the normal force and net horizontal force.
A coefficient of friction is a term in physics use to describe the resistant force acting on an object due to its normal force and the two surfaces that are in contact.
Static friction is the force between stationary objects and dynamic friction is the force between moving objects.