Legend

 Fnet = net force m = mass a = acceleration Fg = force of gravity FN = normal force Ff = force of friction
This skier moves ahead by pushing back on the ground with the pole.  The ground pushes forward on the pole, which transmits the force to the person.  Without friction, it would be hard to move around or stop.

Friction

Most of the time we ignore or assume many things, for example, we ignore air resistance, assume Fg is constant.  However, friction is too significant to completely ignore, and is a factor in many problems.

There are two types of friction, and for good reason, they are different.  For example, when you brake hard and the tires do not skid, friction helps you to stop.  However, if you brake hard and the tires skid, you still eventually stop, even though it takes longer.

These are the two frictions:

 Static Friction Force that acts as long as surfaces in contact are not slipping. Kinetic Friction When surfaces in contact are slipping, this is the force acting on it.

Direction of friction force is always parallel to the surface of contact, and in the direction opposite to applied forces.
Size of friction force depends on normal force (FN) and force of gravity (Fg).

 Ff = FN = coefficient of static friction Ff = FN = coefficient of static friction

The coefficients of friction are a measure of the amount of cling a surface has.  You may notice that when pushing something heavy the toughest part is getting it to move; once it's sliding along, it is easier to push.  This is reflected in and
. For most surfaces, > (surfaces stick to each other better when they aren't sliding).

 Example:
 A horizontal force F acting through a spring scale is applied to a motionless block of weight Fg.   Static friction opposes the impending motion with a force Ff that can be as much as Ff (max) = FN The free-body diagram indicates that no horizontal acceleration will occur until F > Ff.
 Weight Inclined Plane

 created by Will Kuo and Stan Watterson thinkquest participants  team 25844 ©1999 Eloquent Logic. All rights reserved.