Fluids - Pascal's Principle / Negative vs Positive

In summary, the problem involves calculating the weight that can be lifted by a hydraulic jack with specific dimensions and a maximum force of 110 N. A question is raised about the textbook's notation of forces as positive and negative, and a potential explanation is given.
  • #1
crono_
44
0

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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  • #2
Any thoughts?
 
  • #3
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
 

1. What is Pascal's Principle?

Pascal's Principle, also known as the principle of transmission of fluid-pressure, states that a change in pressure at any point in an enclosed fluid will be transmitted equally to all points in the fluid.

2. How does Pascal's Principle apply to fluids?

Pascal's Principle applies to all fluids, including liquids and gases. It states that a change in pressure at one point in an enclosed fluid will be transmitted equally to all other points in the fluid, regardless of the shape or volume of the container.

3. What is the difference between negative and positive pressure?

Negative pressure, also known as vacuum or suction, refers to a pressure that is lower than the surrounding atmospheric pressure. Positive pressure, on the other hand, refers to a pressure that is higher than atmospheric pressure.

4. How does negative pressure affect fluids?

Negative pressure can cause fluids to be drawn into a container or a vacuum. In other words, the fluid will move from an area of higher pressure to an area of lower pressure.

5. How does positive pressure affect fluids?

Positive pressure can cause fluids to be pushed or squeezed out of a container. In other words, the fluid will move from an area of higher pressure to an area of lower pressure.

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