# Friction

Photo by: Thomas Otto

Friction is a force that resists motion when the surface of one object slides over the surface of another. Frictional forces are always parallel to the surfaces in contact, and they oppose any motion or attempted motion. No movement will occur unless a force equal to or greater than the frictional force is applied to the body or bodies that can move.

Friction is often regarded as a nuisance because it reduces the efficiency of machines. It is also, however, an essential force for such items as nails, screws, pliers, bolts, forceps, and matches. Without friction we could not walk, play a violin, or pick up a glass of water.

Gravity and friction are the two most common forces affecting our lives. While we know a good deal about gravitational forces, we know relatively little about friction. Frictional forces are believed to arise from the forces of attraction between the molecules in two surfaces that are pushed together by pressure.

The surface of a given material may feel smooth. But at the atomic level, it is filled with valleys and hills a hundred or more atoms or molecules high. When one object is placed on top of another object, the pressure it exerts squeezes the hills and valleys in the two surfaces together. Molecules on the two surfaces tend to stick to one another, producing friction.

## The laws of friction

Three laws apply to frictional forces. First, the force of friction between an object and the surface on which it rests is proportional to the weight of the object. The heavier the object, the greater the frictional force. The lighter the object, the less the frictional force. Second, in most cases, the force of friction between an object and the surface on which it rests is independent of the surface area of the object. Two objects may have very different surface areas, but if they weigh the same amount they will exert the same frictional force. Third, the force of friction between an object and the surface on which it rests is independent of the speed at which the object moves—provided that the speed is not zero.

The third law suggests that two kinds of friction exist, static and kinetic friction. Static friction is the force required to make an object at rest begin to move. In contrast, kinetic friction is the resistance to motion of an object moving across a surface. Static friction always is greater than kinetic friction because fewer areas of the object are in contact with a surface once a body is in motion. Molecules are not in contact long enough to form attractions to each other.

Two kinds of kinetic friction exist: sliding and rolling friction. When you push a book across a table top, the resistance you experience is caused by sliding friction. When you push a wheelbarrow over the ground, the resistance you feel is caused by rolling friction. In general, rolling friction is always less than sliding friction. One reason is that the contact area between a wheel and a surface is very small and brief.

The friction between two surfaces can be reduced by using a lubricant, such as oil. The oil reduces the amount of contact between two surfaces and allows them to slide or roll over each other more smoothly.

## User Contributions:

bob
p.s. thank you on my report on friction and such i will surely get an a+ thank you again
KRIZZA
Your Frction was really good!!!..I'm sure that i can learn a lot of things from your report..it will also help me to improve my weaknesses in my studies...thank you very much!!!
Crazy Girl
You are not quite correct by stating that the frictional force is proportional to the weight of the sliding object. The frictional force is proportional to the pressure that exists between the two surfaces i.e. weight divided by area. Take a book and slide it across a table and measure the frictional force (say a spring scale parallel to the table surface). Then cut the book in half and measure again. The frictional force will be the same. Why? The area was halved as well as the weight which gives us the same pressure. This example satisfy two of the rules of friction:
1. The frictional force is proportional the pressure between the surfaces in contact.
2. The frictional force is independent to the extent of the surfaces in contact (the area).
The friction is equal to sloping of block in a plane?
anitha
can u make it clear how friction is independent of area of contact?
which type of surface is friction-less between wheel and wedge?
Kate
Thank you. That helped me so much.
Now I know where to go when I need help
Kate
Thank you. That helped me so much.
Now I know where to go when I need help
Anonymous
Great website and very helpful in terms of giving and taking in information. And has understandable language unlike most sites these days, sometimes you find that you can't understand a thing even though google itself and the sites are supposed to give you the help you need in understandable terms