Fluids - Pascal's Principle / Negative vs Positive

AI Thread Summary
The discussion revolves around a problem involving a hydraulic jack and the application of Pascal's Principle. The user expresses confusion regarding the signs of forces in the textbook, questioning why F is considered positive while F1 is negative, despite both pointing downwards. A possible explanation is suggested, indicating that the textbook may be summing moments about the hinge and defining counterclockwise moments as positive. The user also expresses concern about copyright issues related to sharing a diagram from the textbook. Ultimately, the conversation highlights the complexities of understanding force directionality in hydraulic systems.
crono_
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Homework Statement



The handle of a hydraulic jack is 15 cm long and is pivoted 2.5 cm from the input piston which has a radius of 0.60 cm. The output piston has a radius of 1.2 cm. What weight could be lifted by the jack if the person pushing on the handle is to exert no more than 110 N of force?

http://edugen.wiley.com/edugen/courses/crs2216/cutnell3550/ssg/cutnell3255c11/image_n/nw0002-y.jpg

The Attempt at a Solution



Silly question...I doubt I'm right, as I'm hardly an authority to go up against a textbook. But that being said...

I'm confused as to why the textbook (Cutnell & Johnson Physics Study Guide 7th ed.) says that F is positive when F1 is negative? They're both pointing down...so...aren't they both negative? F2 is positive and pointing upwards, that makes sense. But why F is also positive completely eludes me...

Am I missing something completely obvious?

On a side note - I hope I'm not violating any copyright laws by posting that diagram. I just want the answer, not a prison term!
 
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Any thoughts?
 
crono_ said:

Homework Statement



The handle of a hydraulic jack is 15 cm long and is pivoted 2.5 cm from the input piston which has a radius of 0.60 cm. The output piston has a radius of 1.2 cm. What weight could be lifted by the jack if the person pushing on the handle is to exert no more than 110 N of force?

http://edugen.wiley.com/edugen/courses/crs2216/cutnell3550/ssg/cutnell3255c11/image_n/nw0002-y.jpg


The Attempt at a Solution



Silly question...I doubt I'm right, as I'm hardly an authority to go up against a textbook. But that being said...

I'm confused as to why the textbook (Cutnell & Johnson Physics Study Guide 7th ed.) says that F is positive when F1 is negative? They're both pointing down...so...aren't they both negative? F2 is positive and pointing upwards, that makes sense. But why F is also positive completely eludes me...

Am I missing something completely obvious?

On a side note - I hope I'm not violating any copyright laws by posting that diagram. I just want the answer, not a prison term!

Perhaps they are summing the moments about the hinge in order to determine the force F1. So maybe they chose the + moment in the CCW direction (making F x d positive). Then the reaction at F1 (F1 x (d-2.5)) would be negative since it would cause a CW rotation.

Just a thought.

CS
 
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