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copied!<p>Laurens, the physics is much simpler if you stay in two dimensional space. In 2D space, rotations are described by a scalar, resistance to rotation (moment of inertia) is described by a scalar, and rotations are additive and commutative. Things get hairy (much, much hairier) in three dimensional space.</p> <p>When you connect two objects, the combined object has its own center of mass. To calculate the moment of inertia of this combined object, you need to sum the moments of inertia of the individual objects and also add on offset term given by the <a href="http://en.wikipedia.org/wiki/Parallel_axis_theorem" rel="nofollow noreferrer">Steiner parallel axis theorem</a> for each individual object. This offset term is the mass of the object times the square of the distance to the composite center of mass.</p> <p>The primary reason you need to know the moment of inertia is so that you can simulate the response to torques that act on your object. This is fairly straightforward in 2D physics. Rotational behavior is an analog to Newton's second law. Instead of F=ma you use T=Iα. (Things once again are much hairier in 3D space.) You need to find the external forces and torques, solve for linear acceleration and rotational acceleration, and then integrate numerically.</p> <p>A good beginner's book on game physics is probably in order. You can find a list of recommended texts in <a href="https://gamedev.stackexchange.com/questions/16362/what-is-a-good-book-on-physics-for-game-development">this question at the gamedev sister site</a>.</p>

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