GRADES 3-8
RESISTANCE IS FUTILE

BACKGROUND INFORMATION:

The science behind how things move when forces are applied (called physics) is of great importance to engineers. Statics and dynamics are the branches of physical science that describe, define and explain the relationships between forces, energy, and motion. Statics is the science of equilibrium forces, where there is no motion and all the forces are balanced. Dynamics is the science of objects in motion.

Forces are pushes or pulls on an object. A force is a vector; it has both a magnitude (a number value) and a direction. The fact that forces have directions becomes very important in the study of statics and dynamics. For example, if you push straight down on a book on a table, no matter how hard you push down, the book will not move to the side along the table. The force downward is balanced by an equal and opposite force up from the table. The book is in static equilibrium. If you push hard on the side of the book parallel to the table, however, it will move across the table! It is no longer in equilibrium. The force is usually created through some type of energy. Energy can take many forms such as work, heat, or chemical. In the book example, you provided the energy to push the book.

When a force is applied to an object to push it across a surface, one of two things will happen. Either the object will begin to move across the surface, or it may stay in place, even though there is a force pushing on it. When a force is applied to the side of an object (as if to move it across the surface), a friction force is generated between the object and the surface.

Friction works against the applied force. It resists the applied force and the possibility of motion. If the applied force is larger than the friction force, the object will move across the table. Interestingly enough, it takes a larger force to set an object in motion than it does to keep the object moving! It is possible that when you tried to flick the coin across the carpeted surface, it may not even have moved. The force you applied was not large enough to overcome the friction.

Engineers and scientists must be able to calculate friction forces. The friction force is related to the weight of the object and the type of surface it will slide across. Weight is a force that is equal to the mass of the object (the number of molecules) times the gravitational acceleration, g. Weight is a vector that points downward into the center of the earth. It is measured in kilogram meters per second squared, or a Newton, in the metric system, or pounds force in the English system.

A parameter called the coefficient of friction is used to calculate the effect a surface has on motion. A very smooth surface like ice has a very small coefficient of friction, so the friction force is also very small. As a surface gets rougher, the coefficient of friction increases and so does the friction force. A wooden surface is rougher than ice, but not as rough as the sandpaper. The coin across the wooden surface will have a greater friction force than the coin across the ice does, but it will experience less friction than the coin across the sandpaper. Likewise, the carpeted surface is even rougher than the sandpaper, so of our examples, it should have the largest coefficient of friction and friction force.

Pushing A Crate Over An Icy, Rough, And Carpeted Surface Requires Different Amounts Of Force

In order to help us calculate the friction force, engineers and scientists use a drawing called a free body diagram to help us picture the forces on an object. Using the crate example from the above images, a free body diagram would look like the image below.

In this diagram, we show that the crate has a weight acting downward. There is an equal and opposite reaction force called the normal force where the crate touches the surface. The applied force is shown on the left, and the friction force (in the opposite direction to the applied force) is drawn at the bottom of the crate, again where it touches the surface.

The surface is not shown; only the object with all the forces acting on it. That's why it is called a free body diagram! The values of the applied force, the weight, and the normal force are all known, but we need to calculate the friction force.

The friction force is equal to the coefficient of friction times the value of the normal force (which is equal to the weight!). The coefficients of friction are determined from experiments and can be found in engineering reference books for a wide range of surfaces.

There are actually two coefficients we can use. One is called the coefficient of static friction, and the other is called the coefficient of kinetic friction. If the crate is at rest, or not moving, the friction force is calculated with the coefficient of static friction times the normal force. If it is already moving, the friction force is computed with the slightly smaller coefficient of kinetic friction. As was stated up at the start of this section, it takes a larger force to start an object moving than to keep it moving! Friction resists movement!