Kim,

In the simplest terms a force is a push or a pull. Which term is used depends upon direction. If you apply a force toward you it is called a pull; if applied away from you it's a push.

The law of inertia (sometimes referred to as Galileo's Principle of Inertia [he got it first] or Newton's First Law of Motion) simply says that any mass is going to continue to do what it's already doing - if it's at rest, it's going to stay that way; if it's moving, it's going to continue to do that, at a constant speed and in the same direction - until something happens to change what the mass is doing.

What has to happen? Something must push or pull the mass. In other words, a force has to act or be applied. Depending on the direction of the force, the mass may speed up, slow down, and/or change direction. So, as stated by Newton's Second Law of Motion, any force (however small) can change the motion of any mass (however large).

The confusion arises because on earth we are stuck with an ever-present force called gravity which complicates things. There are also other forces which can muddy the waters. For example, Aristotle's conclusion that a force is necessary to _maintain_ motion seems obvious. Every little kid who's ever pushed a toy car across the floor knows that when the pushing stops the car soon comes to rest.

Galileo's argument was that forces _CHANGE_ motion, so that the car stops because the total force on it is NOT zero when the kid lets go. When the little kid is pushing the car, there are TWO forces acting on it. (Actually there are more, including gravity, but they don't affect the motion _across_ the floor.) Those two forces are the kid and one we call friction. That's the one that Aristotle and apparently everybody else missed for a looooong time. While the kid is pushing the car at constant speed his force and friction, acting in opposite directions, give a total force of zero, so there are no "unbalanced" forces, and no change in motion. When the kid lets go, the friction remains and is the "unbalanced" force which changes the motion of (stops) the car.

Newton's Third Law of Motion is the most widely misunderstood of them all. Most simply stated it means that you can't push on anything without it pushing back on you.

The key to simplifying the understanding of dynamics (Newton's Laws) is called a "free-body diagram". This is a sketch of the object we wish to observe and all of the forces which are applied TO that object. The forces applied BY the object to its environment (the so-called "reaction" forces) can have no effect on the object's state of motion.

I hope this helps.