Purpose: To demonstrate the object collision feature of Blender's Game Engine.
Introduction to Object CollisionEdit
In this tutorial, you will learn the basics of object collision within the Blender Game Engine (BGE). These physics simulations can be used for many things, from Blender games to use in animations, the bullet physics engine offers a massive number of possibilities, limited only by your imagination (and the amount of RAM your computer possesses). The tutorials found within this wikibook on the subject of the BGE are generally focused on game creation, but the concepts taught within them can be applied to a multitude of situations.
With the mouse cursor over a 3D viewport, press NUMPAD-7 to switch to TOP view.
Press the spacebar and select Add >Mesh >Plane. Press TAB to leave edit mode and enter object mode. 
Enlarge the plane by pressing S and dragging the mouse cursor away from the center of the plane. Click to stop re-sizing. The plane will serve as the 'floor'.
Add A UVSphere using the same sequence for adding the plane object. Press TAB to enter object mode.
With the mouse cursor over a 3D viewport, press NUMPAD 1 for Front view, then press G and move the sphere above the plane. Click to exit Grab Mode. You may need to scale the sphere down. It will be the ball which collides with the floor.
Go to logic panel (move cursor over Button Viewport and press F4).
Click on actor button (in the upper left corner of the logic panel), then Dynamic, then Rigid Body. This activates physics properties for the sphere so it can bounce.
Change the value of "Radius" to 2.0. Notice the dotted line that appears around the sphere. This dotted line marks how big the physics engine thinks this sphere is. For now, set it back to 1.0.
Next, you want to add some colour to the sphere and the plane (see Blender_3D:_Noob_to_Pro/Quickie_Material for details), so you can tell the two apart during simulation.
Move the cursor over a 3D viewport and press P. The sphere will drop down and collide with the plane. Press ESC to end simulation
For fun, try rotating the plane in front or side (NUMPAD 3) view and press P again. The ball will first hit the plane, then roll down. Add several more planes rotated to differing angles and position them in the falling path of the ball to keep it going.
This YouTube tutorial might also help: Link.