Dawn Michelle Todd, age 9, of Lewisville, N.C., for her question:
WHAT EXACTLY IS FRICTION?
Friction is usually an unwanted source of heat because it may damage an object. In a machine, for example, heat can be created as moving parts rub against each other, and this may cause the parts to wear down. It is for his reason that oil is used between a machine’s moving parts. The oil reduces the friction and thereby decreases the heat.
Friction is a force that makes two objects placed next to each’other resist being moved across one another. In other words, it is the resistance to motion of surfaces that touch.
If you have one box standing on top of another, the one on top can be lifted without any resistance except that of gravity. However, if you tried to push the top box off the lower one, you would be faced with the extra resistance caused by friction.
Friction has a lot of good things going for it. A locomotive, for example, can thank friction for making its wheels grip the rails of the track. Friction also lets a conveyor belt turn on pulleys without slipping.
Did you know that you would not be able to walk without friction to keep your shoes from sliding on the floor? You can prove this when you try to walk on an icy sidewalk. The slick ice produces less friction than a floor, and this allows the shoes to slip.
But friction also has some disadvantages. Friction makes heat, and heat often is the cause of wear. Lubricants are used to fill the space between the two moving parts and thereby cut down on the amount of heat produced.
There are three kinds of friction. Sliding or kinetic friction develops when two surfaces slide across each other, such as when you push a book across your desk. A second type of friction is called rolling friction. An example of this is a car’s tire rolling down the street.
A third type is called fluid friction or viscosity. With this type of action, a fluid is placed between solid objects. Thinner fluids have less viscosity than thicker fluids, and they usually flow faster.
A basic law of friction states that the force needed to overcome friction is proportional to the total normal, or perpendicular, force pressing one surface against the other. That means that when the weight of a box being pulled across a floor is doubled, the force necessary to pull it must be doubled.
Friction’s basic law goes on to state that when a box weighs four times as much as the one you originally doubled in weight, four times as much force must be used to pull it. The ratio between the weight being moved and the force pressing the surfaces together is called the coefficient of friction, or C.F. The value of C.F. depends on the type of surfaces moving against each other. The coefficient of friction equals the force needed to move an object divided by the force pressing the surfaces together.