# Finding the tension of a string

picure is http://home.earthlink.net/~suburban-xrisis/clip2.jpg [Broken]
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 dont see how it is 118??

Last edited by a moderator:

## Answers and Replies

dextercioby
Homework Helper
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.

learningphysics
Homework Helper
UrbanXrisis said:
picure is http://home.earthlink.net/~suburban-xrisis/clip2.jpg [Broken]
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 dont 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

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 dont 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 dont 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 isnt 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 $\frac{1}{2}MR^2$ ...again, though, in this case you don't really need it.)

^^ That gap is the strangest thing...

Last edited: