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Homework Help: What is the mass of the meter stick?

  1. Nov 28, 2004 #1
    A meter stick blances horizontally on a knife-edge at the 50.0 cm mark. With two 5.00 g coins stacked over the 12.0 cm mark, the stick is found to balance at the 45.5 cm mark. What is the mass of the meter stick?

    I know it is going to be having equal torques. The torque on one side should be .01 kg*9.8 m/s^2* .335 m = .03283 N*m + the torque of the .335 m of meter stick. The other side is just the torque of the .455 m of stick. I'm not sure how to calculate the torque of the meter stick.
  2. jcsd
  3. Nov 28, 2004 #2
    The answer is supposed to be 74.4 g.
  4. Nov 28, 2004 #3
    Pick a point, any point, and sum moments to zero. Define your variables; mass of stick and coins, ms and mc; forces, a, b, and g. Solve for ms.
  5. Nov 28, 2004 #4
    The stick's centre of mass experiences a weight force mg, (50.0 - 45.5) cm from the knife-edge, producing a torque of mg(0.045 m). This torque is balanced by the torque of the coins (0.01000 kg)g(0.335 m) = mg(0.045 m). Therefore m = 74.4 g.
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