Acceleration: Changes in Both Speed and Direction

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Acceleration often involves both a change in speed and a change in direction. Changing both components of velocity results in a curved path of motion. In these cases, the acceleration vector is the sum of two parts (components). One part, the tangential acceleration, acts along the direction of motion, parallel to the velocity, resulting in a change of speed. The other part, the radial acceleration, acts perpendicular to the direction of motion, resulting in a change of direction. In order to change the speed of an object moving in a circle, for example, one needs some acceleration along the direction of motion, in addition to the component of acceleration in the radial direction (pointing to the center) that keeps the object moving in a circle. In the case of a space shuttle in orbit, the radial acceleration is the force of gravity pulling the shuttle toward Earth, while a tangential acceleration is achieved by firing rockets along the direction of motion.

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