Physics Question

C

cancel2 2022

Canceled
Sensible answers only!

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Because the direction of the forces on the table are horizontal I'll say zero . 100N in one direction - 100N in the other.
 
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Evmetro said:
It won't look like zero if you cut the rope...
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Sure it would. If you look at the drawing there's no stabilizing force supporting the scale, e.g. a post secured to the table. Cut the rope, both weights fall to the ground, and the scale will still read zero.
 
aloysious said:
I'll be on a flight to Hawaii on Saturday. Hope the answer doesn't get lost! Are there any assumptions you're leaving out? E.g., what kind of scale this is?
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OK, I'll post the answer tomorrow night around 10pm GMT. This is not a trick question, it's just pure physics.

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aloysious said:
Sure it would. If you look at the drawing there's no stabilizing force supporting the scale, e.g. a post secured to the table. Cut the rope, both weights fall to the ground, and the scale will still read zero.
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Actually this caused me to change my mind.
The scale in the drawing would read 100N.
 
Mott the Hoople said:
I would say that you have a 100 N weight holding the scale stationary and another 100 N weight pulling on the spring so using Newton's second law I'd say the scale would read 100 N.
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I'm tending toward that answer myself...although it DOES seem counter-intuitive.

Each time I think it over...I come away favoring a new answer...with zero coming up more often than I feel comfortable with. My neck is getting sore trying to picture the "scale" hanging from a hook in the ceiling.

MY FINAL ANSWER:

Beats the shit out of me.
 
What trips most people up on this is they visualize this as two forces operating equally in opposite directions but that's not the case. the first 100 N weight is holding the stationary end of the scale stationary. It would be the same if you were holding the the string of the stationary end of the scale with your arm. You wouldn't count the force of your arm on the stationary end, would you? So since the 100 N weight is holding the stationary end of the scale stationary the second weigh would only exert 100 N force on the spring end of the scale. Therefor the scale would read 100 N. In other words the weight on the stationary end isn't exerting a force on the scale's spring. It's just holding it stationary.
 
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OK, here is the answer and the explanation.

The scale is reading the tension in the string and that is 100 N.

Nothing is moving or accelerating, so the net force on the spring is zero. Likewise, the net force on either of the 100 N weights is also zero. The spring scale does not measure the net force however, it simply measures the tension exerted by the string. If you were to hold the spring balance vertically you would know the answer without thinking.
 
Mott the Hoople said:
What trips most people up on this is they visualize this as two forces operating equally in opposite directions but that's not the case. the first 100 N weight is holding the stationary end of the scale stationary. It would be the same if you were holding the the string of the stationary end of the scale with your arm. You wouldn't count the force of your arm on the stationary end, would you? So since the 100 N weight is holding the stationary end of the scale stationary the second weigh would only exert 100 N force on the spring end of the scale. Therefor the scale would read 100 N. In other words the weight on the stationary end isn't exerting a force on the scale's spring. It's just holding it stationary.
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I think you've got it...and Moon seems to say it is so.

Damn...there was a time when I would get something like that.

Getting old is a bitch.
 
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