Atwood's machine is simply a pair of masses hung by a string which goes over a pair of pulleys.

Draw freebody diagrams of the two masses in the space below. Use arrows to label all the forces on each mass, and label each arrow.

freebody-diagram of M1 freebody diagram of M2

Forces on mass M1: Forces on mass M2: force X Y force X Y ------------------------------------ ---------------------------------- | | | | ------------------------------------ ---------------------------------- | | | | ------------------------------------ ---------------------------------- | | | | ------------------------------------ ---------------------------------- total | total | | | ------------------------------------ ----------------------------------

Write down the Newton's Second Law in the Y-direction for each mass. Use "a1" as the acceleration of mass M1 in the Y-direction, and "a2" as the acceleration of mass M2 in the Y-direction.

total F = M1 * a1 total F = M2 * a2

You should have two equations, one for the motion of block M1, one for the motion of block M2. Suppose that I tell you the values for the masses, M1 and M2. How many unknown quantities are left in the two equations?

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