Categories: Guide to Space

# What is the Coefficient of Friction?

Ever watch a car spin its wheels and notice all the smoke and tire marks it leaves behind? How about going down a slide? You might have noticed that if it were wet, you travelled farther than if the surface was dry. Ever wonder just how far you’d get if you tried to slide on wet concrete (don’t this, by the way!). Why is it that some surfaces are easy to slide across while others are just destined to stop you short? It comes down to a little thing known as friction, which is essentially the force that resists surfaces from sliding against each other. When it comes to measuring friction, the tool which scientists use is called the Coefficient of Friction or COH.

The COH is the value which describes the ratio of the force of friction between two bodies and the force pressing them together. They range from near zero to greater than one, depending on the types of materials used.For example, ice on steel has a low coefficient of friction, while rubber on pavement (i.e. car tires on the road) has a comparatively high one. In short, rougher surfaces tend to have higher effective values whereas smoother surfaces have lower due to the friction they generate when pressed together.

There are essentially two kind of coefficients; static and kinetic. The static coefficient of friction is the coefficient of friction that applies to objects that are motionless. The kinetic or sliding coefficient of friction is the coefficient of friction that applies to objects that are in motion.The coefficient of friction is not always the same for objects that are motionless and objects that are in motion; motionless objects often experience more friction than moving ones, requiring more force to put them in motion than to sustain them in motion.

Most dry materials in combination have friction coefficient values between 0.3 and 0.6. Values outside this range are rarer, but teflon, for example, can have a coefficient as low as 0.04. A value of zero would mean no friction at all, which is elusive at best, whereas a value above 1 would mean that the force required to slide an object along the surface is greater than the normal force of the surface on the object.

Mathematically, frictional force can be expressed asFf= ? N, where Ff = frictional force (N, lb), ? = static (?s) or kinetic (?k) frictional coefficient, N = normal force (N, lb).

We have written many articles about the coefficient of friction for Universe Today. Here’s an article about friction, and here’s an article about aerobraking.

If you’d like more info on the Friction, check out Hyperphysics, and here’s a link to Friction Games for Kids by Science Kids.

We’ve also recorded an entire episode of Astronomy Cast all about Gravity. Listen here, Episode 102: Gravity.

Matt Williams

Matt Williams is a space journalist and science communicator for Universe Today and Interesting Engineering. He's also a science fiction author, podcaster (Stories from Space), and Taekwon-Do instructor who lives on Vancouver Island with his wife and family.

Share
Matt Williams

## 2024 Perseids Light Up the August Sky

That ‘Old Faithful’ of meteor showers the Perseids peaks early next week.

6 hours ago

## Habitable Planet’s Orbiting Red Dwarf Suns Could at Risk from Far-Ultraviolet Radiation

The question of whether or not red dwarf stars can support habitable planets has been…

23 hours ago

## Starlink Direct-to-Cell Satellites are Coming. What Will Be Their Impact on Astronomy?

Mention the name Starlink among the astronomy community and you will often be greeted with…

24 hours ago

## Fast-Tracking the Search for Habitable Worlds

Modern astronomy would struggle without AI and machine learning (ML), which have become indispensable tools.…

1 day ago

## Astronomers See Planets Forming Around Binary Stars

Over 5,000 exoplanets have been discovered around distant star systems. Protoplanetary disks have been discovered…

1 day ago

## China's Lunar Samples Contain Graphene Flakes

In 2004, scientists at the University of Manchester first isolated and investigated graphene, the supermaterial…

4 days ago