# Mechanics (Finding Reaction Force)

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#1
Here is the question.
"A ring of mass 0.2 kg is threaded on a fixed rough horizontal rod and a light inextensible string is
attached to the ring at an angle ! above the horizontal, where cos ! = 0.96. The ring is in limiting
equilibrium with the tension in the string T N (see diagram). Given that the coefficient of friction
between the ring and the rod is 0.25, find the value of T."
I assume from this text, you can imagine how it would look like. I have a question!
So, i know how to solve this but what i don't get is finding the reaction force. In this case, the reaction force by the tension in the string is going to act UPWARDS while the weight is acting DOWNWARDS so, you have to subtract them. But, how do you know which is bigger? Do you always have to assume which is bigger and then, try to solve the problem? Or is there a theoretical concept which can explain this?
Thanks a lot!!
0
8 months ago
#2
(Original post by Willi Yan)
Here is the question.
"A ring of mass 0.2 kg is threaded on a fixed rough horizontal rod and a light inextensible string is
attached to the ring at an angle ! above the horizontal, where cos ! = 0.96. The ring is in limiting
equilibrium with the tension in the string T N (see diagram). Given that the coefficient of friction
between the ring and the rod is 0.25, find the value of T."
I assume from this text, you can imagine how it would look like. I have a question!
So, i know how to solve this but what i don't get is finding the reaction force. In this case, the reaction force by the tension in the string is going to act UPWARDS while the weight is acting DOWNWARDS so, you have to subtract them. But, how do you know which is bigger? Do you always have to assume which is bigger and then, try to solve the problem? Or is there a theoretical concept which can explain this?
Thanks a lot!!
Assuming positive is upwards.
reaction + resolved tension = weight
If the resolved tension was greater than the weight, then the reaction is negative and acts downwards. The maths works out without knowing which way the reaction works.
Last edited by mqb2766; 8 months ago
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