What is A Force?

To date, we have talked about motion in a straight line, but we haven't really talked about what causes a change in motion. Now we need to define a new term: Force Simply stated, a force is a push or a pull. Often, a force will cause a change of velocity on an object. Forces can cause stationary objects to start moving, and moving objects to change their speed and direction. We will discuss all of this in great detail in later lessons, but first we want to get some basics.

The SI unit for force is the Newton, named for Sir Isaac Newton. A Newton (abbreviated N) is actually the product of mass times acceleration (in a round about sort of way) and can further be broken down as:

1 N = 1 kg.m/s2

1 Newton has about the same weight equivalent as a MacDonald's Quarter Pounder without the catsup, mustard, pickle, or bun. Pre-cooked weight, of course. In the CGS system, the unit of force is the Dyne and in the British System, the unit of force is the pound (lb). There are basically two types of forces: contact forces and field forces. Contact forces are forces that require physical contact between two objects. Examples are kicking a ball or friction. Field forces are forces that can act over a distance without any physical contact, such as magnetic force or gravity.

Forces are vectors - they have both magnitude and direction. When several forces act upon an object, we add all of the force vectors to determine a single resultant, or net force that is acting on the object. One of the most useful tools in Physics used for solving force problems is the Free Body Diagram. A free body diagram is used to isolate the object and show only the forces acting upon that object. They are easy to draw - we use a simple dot to represent the center of the mass of the object and vector arrows pointing in the appropriate direction to show the forces.

Consider our plane at the right. There is a force pulling it down (gravity, which we will normally call weight), a lifting force caused by the air passing around the wings, a forward driving force (thrust) caused by the engines, and some air resistance working against the plane's motion. We would represent the free body diagram as shown. Free body diagrams will make our work much easier.

Here are a couple of applets you can play with. Try

Equilibrium of 3 Forces

Resultant of Forces (Addition of Vectors)