SNHS Physics Blog 4: Friction

Yunhui Shim

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When we push a food tray across the cafeteria table, and see it come to a stop, we know that we are experiencing friction. Frictional forces resist motion, and even “smooth” surfaces have irregularities that when viewed with a microscope, seem clear. This type of roughness contributes to “friction”. Sliding through a surface requires a set amount of force to overcome the resistance that is caused by the microscopic hills and valleys that bump against one another. The force that opposes the motion of one surface over the other is referred to as friction. Most oftentimes, it is assumed that reducing friction is the most desirable. For instance, the 20% of the gasoline in our car does nothing but overcome the friction within the car’s engine. However, friction can be quite helpful in some other cases. For example, if you decide to walk forward after standing still on a surface, the force that accelerates one is the force of friction. If you ever walked on slippery ice, you would know how hard it is to walk when friction is reduced, so imagine how difficult it would be if there were no friction!

Kinetic friction is the friction that is encountered when surfaces slide against one another. The force that is designated for kinetic friction is f_k. The f stands for force, and the k stands for kinetic. To oppose motion, this force of kinetic friction must act opposite of the direction of the velocity of the moving object. When attaching a spring scale to a rough object across a table, the scale is what identifies the magnitude of the force of the kinetic friction. Generally, the force of kinetic friction is proportional to the normal force and is derived by the multiplication of the coefficient of kinetic friction to the normal force. μ_k is referred to as the coefficient of kinetic friction, and most typically, the values tend to range from 0 to 1. Rubber on concrete has 0.8 for its μ_k, steel on glass has 0.57, and human joints have 0.003. The properties that kinetic friction has of two sliding surfaces are as follows, as the friction: opposes the motion of the surfaces, proportional to the normal force between surfaces, same regardless of the speed of the surfaces, same regardless of the area of contact between the surfaces.

Static friction is the force that opposes the sliding of one nonmoving surface past another. μ_s is the coefficient of static friction. Like kinetic friction, the static frict ion is due to the microscopic irregularities of the surfaces that are in contact with one another. Static friction, however, is stronger than kinetic friction. The force of static friction is able to have any value ranging from 0 to a defined maximum. For example, waxed ski on snow has a static friction of 0.10, rubber on wet concrete has 0.30, and steel on glass has 0.74. When a brick is on a board, the brick may have no tendency to move while it is level. When the board is slowly tilted, gravity pulls the brick downhill and if the tilt isn’t too great, the brick will stay put. There, however, will come a point when the tilt is increased, and the bricks may lose grip and begin sliding. This is when the force of static friction maxes out, to the point where it is as large as it could be. A stationary object begins moving when the applied force equal the maximum force over the static friction. Once the object is in motion, kinetic friction takes over, and the maximum static friction is to be derived by the coefficient of static friction multiplied to the normal force. In most cases, the μ_s is greater than μ_k.

Applying, what we learned today, it is possible to look into the secret behind the use of the antilock braking systems in automobile technology today. The ABS, or the electronic rotation sensor at each wheel, is what automatically pumps the breaks when the car is about to skid. The wheels in older cars tend to lock when panic breaking, and this is what causes the car to skid, uncontrollably. In general, the sliding or skidding tires are what are subject to kinetic friction, and tires that roll are what are subject to static friction. Therefore, by pumping the breaks, the car is able to continue rotating its’ wheels even when the cars are in an emergency stop, as the static friction is able to determine the stopping distance.