A uniform meter stick of mass 1 kg is hanging from a
thread attached at the stick's midpoint. One block of
mass m= 3 kg hangs from the left end of the stick, and
another block, of unknown mass M, hangs below the
80 cm mark on the meter stick. If the stick remains at
rest in the horizontal position shown above, what is M ?

Correct  M = 5 kg

Explanation:

According to the data, the first block is 50 cm from the center of the stick and the second block is 30 cm from the center of the stick.

Being in balance (horizontal position) their moments are the same:

M · 30 cm = m · 50 cm ⇒ 30 M = 50 · 3 = 150 ⇒ M = 150/30 = 5 kg

M = 5 kg

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