What is Friction?
Most recent answer: 10/22/2007
- Anna Campbell (age 9)
Mexico City, Mexico
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.
(published on 10/22/2007)
Follow-Up #1: friction types
- millie sykes, (age 9)
w. Mike W.
(published on 01/25/2008)
Follow-Up #2: what is the formula for friction?
We have described this problem in many of the previous posts to this question. If you want more details I suggest you visit:
(published on 10/20/2009)
Follow-Up #3: velocity and friction
- Brandon (age 18)
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.
(published on 01/09/2010)
Follow-Up #4: friction and the fundamental forces
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.
* Now we should also add Higgs.
(published on 03/08/2011)
Follow-Up #5: What if there were no friction?
- Leighann (age 14)
(published on 10/24/2011)
Follow-Up #6: types of friction
- k.suresh,lecturer (age 25)
(published on 05/16/2013)