Physics Self-Assessment #108

[TheJourney]

A 0.5-kg uniform meter stick is suspended by a single string at the 30-cm mark. A 0.2-kg mass hangs at the 80 cm mark. What mass hung at the 10-cm mark will produce equilibrium?

A) 0.3 kg
B) 0.5 kg
C) 0.7 kg
D) 1.0 kg

My reasoning was for the meter stick to be at equilibrium the left side must equal the ride side thus: (x)(20cm) = (.2kg)(50cm), solving for x you would get 0.5kg. However, answer choice B is wrong, D is the correct answer. Their explanation states that there is two torques acting on the right side, the .2kg hanging mass and the weight of the meter stick.

How do you figure the weight of the meter stick is acting on the right and not left? Second, they set the torque of the meter stick to (0.5 kg)g(20 cm), "where 20cm represents the distance from the suspension to the center of mass of the meter stick." How did they derive that 20cm was the distance from the center of mass of the meter stick, does that make the meter stick 100cm in length?

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[chenzt]

A 0.5-kg uniform meter stick is suspended by a single string at the 30-cm mark. A 0.2-kg mass hangs at the 80 cm mark. What mass hung at the 10-cm mark will produce equilibrium?

A) 0.3 kg
B) 0.5 kg
C) 0.7 kg
D) 1.0 kg

My reasoning was for the meter stick to be at equilibrium the left side must equal the ride side thus: (x)(20cm) = (.2kg)(50cm), solving for x you would get 0.5kg. However, answer choice B is wrong, D is the correct answer. Their explanation states that there is two torques acting on the right side, the .2kg hanging mass and the weight of the meter stick.

How do you figure the weight of the meter stick is acting on the right and not left? Second, they set the torque of the meter stick to (0.5 kg)g(20 cm), "where 20cm represents the distance from the suspension to the center of mass of the meter stick." How did they derive that 20cm was the distance from the center of mass of the meter stick, does that make the meter stick 100cm in length?

A meter stick by definition is 100cm in length...

 

[TheJourney]

Oh wow, that makes a lot more sense now. Lol thank you.

 

[makingthejump]

I just did this problem without using any formulas and got it correct. If you immediately sum the two torques on the right you get .3. Then just reason your way through it. You know that the torque in left is has a smaller lever b/c it is placed at 10cm. So we know its either C or D. Then just guess by thinking how the test writers developed each answer choice and you likely will not pick C b/c 0.7 seems like a fishy number b/c when added to your torque in the right (.3) you get 1. Seems too easy, I therefore chose D and was correct. Think intuitively like EK teaches for this type of problem