Finding the tension of a string

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    String Tension
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Homework Help Overview

The discussion revolves around finding the tension in a string connected to a pulley system, with specific parameters including the radius of the pulley and the mass of objects involved. The original poster attempts to reconcile their calculated tension with a provided answer from a textbook.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants question the assumptions regarding the ideal nature of the string and the relevance of the pulley radius. There are discussions about the equations used for calculating tension and the implications of the incline's angle on the forces involved.

Discussion Status

Some participants offer alternative perspectives on the calculations and the physical principles at play, suggesting that the original poster may need to reconsider their approach. There is an acknowledgment of differing interpretations of the problem setup, particularly regarding the moment of inertia and its significance.

Contextual Notes

Participants note the potential confusion arising from the assumption of an ideal string versus a non-ideal scenario, as well as the implications of the pulley's mass on the calculations. There is mention of a specific numerical discrepancy between the calculated and expected tension values.

UrbanXrisis
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picure is http://home.earthlink.net/~suburban-xrisis/clip2.jpg
I need to find the Tension on string #1
The radius of the pully is 0.25m of negligible mass

I tried and always got 108N however the book says its 118N

(sin37*15*9.8)+(sin37*15*2)=108N

I don't see how it is 118??
 
Last edited by a moderator:
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It doesn't make any sense...Is the string ideal...?If so,then the tension in it is constant...Why do they give you the radius of the pulley...?Definitely weird...

Anyway,even so,assuming that the wire is not ideal (it would make sense),the II-nd principle for the body on the incline should lead you to the answer...

Daniel.
 
UrbanXrisis said:
picure is http://home.earthlink.net/~suburban-xrisis/clip2.jpg
I need to find the Tension on string #1
The radius of the pully is 0.25m of negligible mass

I tried and always got 108N however the book says its 118N

(sin37*15*9.8)+(sin37*15*2)=108N

I don't see how it is 118??

You equation is wrong. How did you get: (sin37*15*9.8)+(sin37*15*2)
 
Last edited by a moderator:
learningphysics said:
You equation is wrong. How did you get: (sin37*15*9.8)+(sin37*15*2)
Indeed. You have to realize that you're given an acceleration that is already in the direction of motion. There is no need to use sines or cosines on that magnitude, since it's already in component form.

You should wind up with an equation that describes the net forces on the incline's block, which (thanks to Newton) will equate to the mass and acceleration of the block (both of which you know).

Plug and Chug :smile:
 
dextercioby said:
It doesn't make any sense...Is the string ideal...?If so,then the tension in it is constant...Why do they give you the radius of the pulley...?Definitely weird...

Anyway,even so,assuming that the wire is not ideal (it would make sense),the II-nd principle for the body on the incline should lead you to the answer...

Daniel.

the r is to find the moment of inertia of the pully. I actually have some trouble with this. Is there an equation that I can use? I have tried many and don't know why it's not giving the answer of 1.2kgm^2

any ideas?
 
UrbanXrisis said:
the r is to find the moment of inertia of the pully. I actually have some trouble with this. Is there an equation that I can use? I have tried many and don't know why it's not giving the answer of 1.2kgm^2

any ideas?
If there's negligiable mass, then the moment of inertia of the pully isn't significant enough to worry about. I got 118 N using only the information provided in the diagram itself.

(PS - the moment of inertia of a solid cylinder is [itex]\frac{1}{2}MR^2[/itex] ...again, though, in this case you don't really need it.)

^^ That gap is the strangest thing... :confused:
 
Last edited:

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