Physics-based games are designed to mimic the basic laws of physics. Unlike many popular games that do not specifically follow natural law, most physics-based games are designed around Newton's Three Laws of Motion.
The simple short-hand for these laws is:

I. If an object is not moving, it won't start moving unless something else moves it. If an object is moving, it will continue to move unless something else stops it.

II. The amount of force required to move an object is equal to the mass of the object times the acceleration.

III. If something pushes on an object (an action), the object will push back (a reaction).

A physics-based game takes these laws and their applications into account. Games in this category are typically not as graphic-intensive as their main-stream counterparts, but they have made great strides in sophistication over the last ten years. There are quite a few physics-based games available to play for free online.

In the game Magic Pen, for example, the object is to roll a circle to a goal. You can draw two dimensional objects which can be used to push or guide the circle. Dropping a larger object on the circle moves the circle farther than dropping a smaller object onto it from the same height. This is a direct application of Newton's second law. The larger object has more mass and thus hits the object with more force than the smaller object. Dropping two of the same size objects from different heights would yield a similar result. The object dropped from a greater height will develop greater acceleration (due to the simulated effect of gravity), and thus will move the circle farther.

The same example also demonstrates the other two laws. In accordance with Newton's first law, the circle remains at rest until it is acted upon by an external force (the object being dropped on it). Once pushed, the object will roll until gravity, friction, or another force takes a toll. Newton's third law tells us that for the action of dropping the object on the circle, there is a reaction (moving the circle).

As is often the case with physics-based games, the initial level of Magic Pen is very straightforward. Drop one ball on your circle and it will hit the target. But before long, levers and pins find their way into the game. By the third level, players will find themselves in need of some serious engineering to complete the seemingly simple task of rolling their circle to the flag.

Another example of a fun, physics-based game is Ragdoll Cannon 2. The object of this game, as the name implies, is to launch a stick-figure ragdoll out of a cannon at a target. The ragdolls obey the laws of motion, flying through the air until acted upon by an outside force. The flying ragdolls will react differently depending upon the nature of the outside force, which includes walls, floors, trampolines, and ever-present gravity.

Again, the early stages of this game are simple, yet surprisingly addictive. The game rapidly increases in difficulty. I hit the first-level target with my first shot. It took me somewhat longer to move the boulder on the third level so that my ragdoll could find the target there. By the fourth level, I needed to enlist the aid of my seven year-old son to navigate my ragdoll through the narrow spaces between layers to hit the target.

Physics-based games work on both the simple level of an enjoyable diversion and also on the level of creating a great teaching moment for students. Anything that grounds book learning into a practical and fun application is a worthwhile experience for parent and child to share.