Forces and Newton's First Law

What is a Force?

Simply stated, a force is a push or a pull. Often, the application of 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.

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.  Now, how does this relate to Newton's Laws?

First, a little history:

The ancients thought that the natural state of any object was at rest and that, in order for something to move, it needed to have a force applied to it. Galileo realized this wasn't accurate, but it was Sir Isaac Newton's work that quantified the concepts of force and how they affect objects through three very important laws.

The first of these is The Law of Inertia. Newton's First Law states that

An object at rest tends to remain at rest, and an object in motion continues in motion with
a constant velocity, unless the object is acted upon by a net external force.

So what is this inertia thing? Inertia is a property of matter and it is one way we measure an object's mass. Inertia is a resistance to a change in motion. (For an example of inertia and a reason to wear your seatbelt, follow this link: http://www.physicsclassroom.com/mmedia/newtlaws/cci.html) Inertia is why it is hard to start a car rolling by pushing it, and also why it is hard to stop it once you get it going! The more massive an object is, the more it tends to resist a change in motion.

When an object is at rest or moving at a constant velocity, it is said to be in equilibrium. There is no net external force acting on it. In other words, when we add up all of the force vectors, the resultant is zero.

This brings us to a quick side discussion of mass. Newton defined mass as a "quantity of matter" or how much matter is in an object. Today, mass is defined in terms of the inertia of an object, or how hard it is to cause a change in an object's motion. Obviously, physical size does not matter - very large objects can have very little mass, such as a giant styrofoam ball, while very small objects can be incredibly dense and have very large mass. Mass, in the SI system, is measured in kilograms.  Frequently, the term weight is confused with mass. Weight is a term used to describe the force of gravity acting on an object. Weight is a force, given as the quantity of mass times the acceleration on an object due to local gravity. An objects mass will remain constant (unless we add or take away matter) but an object's weight changes depending on its location. For example, an object has a different weight on the moon than on the earth. The mass is the same in both locations, but since gravity is different, the weight is different.