When a body is in uniform circular motion, the force on it changes the direction of its motion but not its speed. For a body moving in a circle of radius r at a constant speed v , its acceleration has a magnitude a = v 2 r and is directed toward the center of the circle. The force required to sustain this acceleration, called the centripetal force, is therefore also directed toward the center of the circle and has magnitude m v 2 / r . Many orbits, such as that of the Moon around the Earth, can be approximated by uniform circular motion. In such cases, the centripetal force is gravity, and by Newton's law of universal gravitation has magnitude G M m / r 2 , where M is the mass of the larger body being orbited. Therefore, the mass of a body can be calculated from observations of another body orbiting around it.