Physics Question from MCAT Physics Book

[notJERRYFALWELL]

THe correct answer is 'C.'

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[PremedMCATer]

Somehow, my original post was lost, so I am retyping it....

My question is from problem 33 on page 138 of the MCAT Physics book concerning force meters. There are two 15 kg masses attached (with ropes) on opposite sides to a force meter and it asks what is the force reading on the force meter. The correct answer is 150 N, but I thought the answer is 0 N (since the left and right forces would cancel). I don't understand how force meters work. If someone can explain this to me, I would really appreciate it. Thanks!

 

[AlternateSome1]

Think of it this way...if you had a rope tied to your right hand and it was attached to your ex's car as he/she is driving away from you after taking your house, money, and kids, and you had your left hand attached to a group of money hungry malpractice lawyers that are pulling the other direction with an equal force, would you feel nothing due to the forces canceling out, or would you feel the combined pulls? Basically, since the ropes are tied to OPPOSITE sides of the force meter, they may cancel each other out and result in no net movement of the meter, but the meter will be under strain. This is a result of tension. The simple force meters are two metal or plastic pieces attached with a spring. You can imagine two ropes pulling a spring from opposite ends and making it stretch (tension), while not causing acceleration of the spring as a whole (forces cancel). I hope this helps.

~AS1~

 

[OnMyWayThere]

So why is the answer 150?

 

[PremedMCATer]

Based on what AlternateSome1 said, shouldn't the answer be 300 N? Because the force meter would "feel" 150 N of pull on each side.

 

[AlternateSome1]

Ok, tension is tricky in the sense that it requires equal forces on both sides of the object. In this case, the equal force is 150N. Tension is equal to the 150, not the combination of both. This doesn't make intuitive sense in the example given, but it does if you consider a mass suspended from a ceiling. If the force exerted downward is 150N, the tension on the rope is 150N. At the same time, there is 150N exerted down at the point of attachement of the mass, there must exist a 150N force up exerted by the attachement point of the string to the ceiling (or else the string would fall). Thus, 150N is exerted on both sides of the rope, the rope is stationary, and the tension is still 150N.

~AS1~