The general theory, inertia, and gravity
The General theory explains that gravity and inertia create the same force that pulls you back. For example, if you were inside a spinning spaceship, you would be pulled to the outer edge, just like gravity was pulling you there. Another example is that if you spin fast, your arms get pulled out from your sides and parallel to the ground. It also states that gravity is not a force, but more of a depression in space caused by objects with a lot of mass, like Earth. (See picture to right)
Inertia, as mentioned with gravity, is similar in the way that they both dictate the resistance of objects. Gravity restricts your ability to move freely in any direction, as it will always pull you down. Inertia causes resistance to objects when they change their state of motion. For example, imagine a ball lying on the ground. In order to get it moving, (changing its state from at rest to in motion) you need to give it a push (the resistance). When a ball stops rolling, that is because of friction being the resistance. With my examples above, about spinning, inertia is in effect there because your change in motion (from standing still to spinning) created a gravity-like effect on your arms. This is called centrifugal [sen-trif-yuh-guhl] force Another fact is the strength of the force of gravity determines your weight. So in the spaceship example above, the faster it spins, the heavier you appear to be, because there is more gravity-like inertia.