Put both of your hands together and move them back and forth. Push your hands together harder and move them faster. What do you experience? Are your hands warming up? Do you hear the sound of the hands moving against each other?
Friction results from the surface of your hands moving in opposite direction to each other. Because your hands are in motion this type of friction is known as sliding friction or kinetic friction .
Another type of friction exists which is known as static friction (). Static friciton is the force that opposes the start of motion. If you have every pushed a car before you have experienced static friction. Initially you really need to push to get the car in motion. Once you have the car rolling it is easy to maintain because the force of friction decreases. The reason you need to push harder at the start is that the four tires are not in motion and they are in contact with the ground. The contact between the tires and the ground has static friction which must be broken to initiate motion. This static friction is known as limiting static frictional force.
The minimum force needed to initiate motion equal to the maximum static frictional force.
Free Body Diagram (FBD) of forces on an object undergoing friction:
The force of friction depends primarily on the force pushing the surfaces together , FN and on the nature of the surfaces in contact.
- Fa = applied force
- Fn = normal force - the upward force that is exerted by a surface which the object rests on
- Ff = force [static or kinetic]
- Fw = weight of object
- Note: All forces above act through the center of mass
Ff = µ·FN
µ (mu) called the coefficient of friction is a constant. µ can never be larger than 1. µ is larger when the surfaces have a large amount of friciton such as concrete and rubber. µ is smaller when the surfaces have s small abount of friciton such as teflon and nylon.
There are many applications which involve materieal that increase or decrease friction. Automotive Technicians deal with lubricants with engines. The use of lubricants reduce the amount of friciton between the moving parts of an engine. Race car drivers smoke/spin their tires prior to the start of a race to heat up the rubber so that the tires become sticky creating a large amount of fricion between the tires and track so that a larger amount of acceleration of the car can result.
Ex 1: A horizontal force of 50N is required to pull an 8.0 Kg block of aluminum at a uniform velocity across a horizontal wooden desk. What is the coefficient of kinetic friciton?
Ex 2: The driver of a 2000 kg car in motion applies the brakes on a dry concrete roadway. Calculate the force of friction between the tires and the road surface. Would the amount of friction be the same if the car was not in motion? If so what would be the difference in friction? (Note: You will need to obtain the coefficient of friction from the table - listed on the right side menu)