Anyone can give me an advice on finding a good program that would allow me to make free-body diagrams, such as a pendulum, a block descending down an inclined plane, a spring attached to a block and other simple graphics. as the ones you see in a typical physics book. I am having difficulties finding such program. I know you may say Adobe Illustrator but that's too expensive and the learning curve is very high, at least for me. Thanks.
Your question is unrelated to TeX, but you may want to take a look at the list of vector graphic editors. For free mathematical drawing, I recommend GeoGebra. For technical drawing, you will obviously need a CAD program. A free alternative to Adobe Illustrator is Inkscape which could be used along with a plug-in to export to PGF/TikZ.
As been stated before, the question is kinda unrelated to TeX, but I would recommend Dia for almost any kind of "simple" figures. For example the box-on-sloping-hill-with-spring scenario.
Dia also has commandline options for export, ie very useful when using Makefiles :)