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. 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

(republished on 07/12/06)

Follow-Up #1

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.  For example 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 01/27/08)

Follow-Up #2

Q:i want to know how many types of frictions exist

-k.suresh,lecturer (age 25)
hyderabad,a.p,india
A:Friction occurs in lots of situations in lots of different ways, not just by the bumpy surfaces Dan discussed. For example, there's a type of friction between a magnet and a moving electrical conductor, caused by eddy currents, even though the magnet and the conductor don't touch. Some old-fashioned phonograph turntables used this to adjust the speed by moving the magnet. There is friction between any object and the fluid (liquid or gas) through which it moves. I don't think that it helps much to try to make a list of some specific number of types of friction, because the categories often overlap.

Mike W.

(published on 01/26/09)

Follow-Up #3

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:   http://en.wikipedia.org/wiki/Friction

LeeH

(published on 10/21/09)

 

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