# What is Friction?

Q:
I need to write what friction is and give some examples. I know some little toy cars are called friction cars. Can you help?
- Anna Campbell (age 9)
Mexico City, Mexico
A:
Anna,
Friction is a force which causes the motion between two surfaces to be reduced. [see below for a more general discussion/ mw] Friction happens because most surfaces are not perfectly smooth. Even a table top which may appear smooth has little bumps in it if you looked at it with a really good microscope. When two surfaces try to move past each other these little bumps collide and slow the motion of the surfaces down causing what we call friction. The rougher a surface is the more and bigger bumps it has and the more friction will affect it. Some examples of this are sliding a wood block down a ramp. If you slide a wooden block down a ramp it is slowed by friction. If you cover the block in sand paper (making it rougher) the block will slide slower because friction is slowing it down more. Friction also increases if you push the surfaces together more. So a full suitcase will have more friction opposing it's motion if you try to slide it across the floor than an empty one. Other examples of friction are very numerous because friction happens any time you move two surfaces that are touching. Try looking around your house and experimenting on your own with rough and smooth surfaces.
Dan

(published on 10/22/2007)

## Follow-Up #1: friction types

Q:
Q is there more than one type of friction
- millie sykes, (age 9)
leicester england
A:
Yes.    Friction is a 'catch all' phrase for any force opposing the relative motion of objects in contact.  Examples include the 'friction' supplied by a parachute in air, or the force opposing the sliding motion of a brick on a table, etc, etc.

LeeH
w. Mike W.

(published on 01/25/2008)

## Follow-Up #2: what is the formula for friction?

Q:
what is the formula for friction?
- bob
A:
There is no 'Formula'.  There are several kinds of friction and several kinds of empirical rules that people have developed to describe friction.
We have described this problem in many of the previous posts to this question.  If you want more details I suggest you visit:

LeeH

(published on 10/20/2009)

## Follow-Up #3: velocity and friction

Q:
Does the velocity of the object have an affect on the magnitude of friction?
- Brandon (age 18)
Milwaukee, WI
A:
There's a special type of friction, between two solid surfaces sliding against each other, for which the friction force is almost independent of the relative velocity. That's often the first sort of friction introduced in physics courses, so students often think that friction is velocity-independent. Even for those sliding surfaces, that's just an approximation. The same courses often teach about static friction, between two surfaces that aren't sliding. It has a higher coefficient, and thus can be a bit bigger than the sliding friction. For things that are sliding very, very slowly, the coefficient of sliding friction goes up toward the static coefficient.

For objects moving slowly through fluids, there's a friction force that's just proportional to their relative velocity. That's probably the simplest type of friction to describe and understand, so in a way it's a shame it isn't the first one taught. Faster motion relative to the fluid leads to more complicated velocity dependence.

Mike W.

(published on 01/09/2010)

## Follow-Up #4: friction and the fundamental forces

Q:
If friction is a force, how does it relate to the 5 Fundamental Forces: STRONG, ELECTROMAGNETIC, WEAK, and GRAVITY?
- teacher
A:

First: I count 4 fundamental forces in your list, unless you're counting electrical and magnetic separately. If you were to do that, however, you'd have to include the magnetic-like part of each of the fundamental forces separately. So let's say you have 3 or 4 forces: gravity, strong nuclear force, and electroweak.* (You might choose to count the electroweak as 1 or 2.)

OK, what about the frictional forces? Generally, these are forces which convert the  kinetic energy of large-scale relative motions to various forms of small-scale random thermal energy. Almost all the familiar visible forces are forms of electromagnetism, and that includes common frictional forces.

I guess that on large scales, gravity plays a role in friction. For example, tidal stresses in the earth and moon are part of the path by which large-scale orbital and rotational energy is converted to thermal energy. The last step, however, in which the energy of big tides is converted to slightly warmer oceans, for example, is always electromagnetic. That's because on the scale of molecules gravity is tiny compared to electromagnetic forces.

Mike W.

* Now we should also add Higgs.

(published on 03/08/2011)

## Follow-Up #5: What if there were no friction?

Q:
What would happen if there is no friction? Can you give me some examples?
- Leighann (age 14)
NJ
A:
I you tried to walk you would fall on your butt;  consider very, very slippery ice.  If you tried to drive a car, first of all you couldn't get it going, second of all if you managed to get it going you couldn't stop.  All of these processes involve friction of one sort or another.

LeeH

(published on 10/24/2011)

## Follow-Up #6: types of friction

Q:
i want to know how many types of frictions exist
- k.suresh,lecturer (age 25)