Atwood machine

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The Atwood machine, or Atwood's machine, consists of two masses connected by a string that extends over a pulley. Rev. George Atwood created this machine in 1784 as a laboratory experiment, and it is commonly used to teach the laws of physics to students today. Unless the masses are identical, they experience uniform acceleration, assuming a frictionless pulley.

The point of the machine is that a demonstration of Newton's law of motion, F=ma, is not easy to see for an object that is simply falling in the Earth's gravity, because the motion is too fast to observe accurately. But in the Atwood machine, the force acting in the two masses is related to the difference of their masses, because they are pulling against each other over the pulley. But the "m" in the equation is the sum of the masses. By making the two masses nearly the same, it is possible to make the motion slow enough that it can be measured easily with a watch and meter stick.