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  "body": "\n## Summary\n\nScrew theory describes the velocities of a rigid body and the forces on it by six-dimensional vectors called screws.\nTwists describe motions, and wrenches describe forces and moments.\nIts geometric basis is Chasles' theorem.\nIt says that every orientation-preserving rigid motion of space is a screw motion.\nThat is, it is a rotation about a line combined with a translation along the same line.\nA screw has an axis, a line in space, and a pitch, the ratio of translation to rotation.\nIt is used in the kinematics and statics of mechanisms and robots.\nThere the power of a wrench on a twist is given by a simple pairing.\n\n## Explanations\n\n### L1 human\n\nTurn a screw with a screwdriver: it rotates and at the same time moves forward along its axis.\nScrew theory says that every motion of a rigid object is like this at each instant: a turn about some axis plus a slide along that axis.\nThe axis and the amount of slide per turn, the pitch, describe the motion completely, up to its speed.\nA pure rotation is a screw of pitch $0$, and a pure translation is a screw of infinite pitch.\nForces work the same way: any system of forces on a body acts like one force along a line together with a twisting moment about that line.\n",
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