Force is one of those physics words that show up a lot in everyday use, and it is one of the ones that is often misused. However, I can’t tell you what force is. I can give you examples of it, such as the gravitational force the earth exerts on everything that has mass, and the normal force the floor exerts on the bottom of your feet, or the force exerted by a spring that is stretched or compressed.

Usually force is mentioned very early in physics courses, as it makes its appearance in Newton’s second law: F=ma, that is, the total force acting on an object is equal to the mass of the object times its acceleration. This is in fact a vector equation, so that direction matters.

Suppose that at this moment you are standing still. This means that your acceleration is zero, and therefore the total force on you is zero. Earth is no doubt still pulling down on you with a force equal to your weight, but that force is evidently being counteracted by an upward force of the same magnitude. This is probably a normal force from the floor or from your chair. Later you may be driving north in your car at 35 miles per hour. At that time the total force on you is still zero – after all, your velocity is not changing, so your acceleration is zero, and by Newton’s second law we know that the total force on you must also be zero.

Later we will deal with what is happening when you travel along a circular path at constant speed; your velocity, which includes direction, is changing all the time. Therefore your acceleration is not zero and there must be a net force acting on you.

Some questions to ponder: What force acts to accelerate you forward when you begin to walk? What is wrong with the classic cartoon moment, when Wile E. Coyote has run past the edge of the cliff, stops, looks down, and then falls?