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# M3 Elastic Springs watch

1. https://gyazo.com/511685f0a4f7e44488188d6121d3a115

I tried equating the weight of the uniform rod to the to the vertical component of the tension in the string as well as using moments about point P, what else should I try?
2. (Original post by moz4rt)

I tried equating the weight of the uniform rod to the to the vertical component of the tension in the string as well as using moments about point P, what else should I try?
Moments about P would do it. Post working if it's not coming out.

Resolving vertically won't work, since you don't know the vertical reaction at P.
3. (Original post by moz4rt)

https://gyazo.com/511685f0a4f7e44488188d6121d3a115

I tried equating the weight of the uniform rod to the to the vertical component of the tension in the string as well as using moments about point P, what else should I try?
Moments about point P are going to be the way to go, as the is a force upwards at P, so equating the weight is not going to give you the actual vertical force at Q.
4. (Original post by Faction Paradox)
Moments about point P are going to be the way to go, as the is a force upwards at P, so equating the weight is not going to give you the actual vertical force at Q.
Do you have the correct answer? I got that T = 2g, but I won't bother explaining if it's incorrect.
Edit: I also got that the natural length = 75/31 = 2.419m.
5. I got the correct answer this time, but instead of using the distance from P to Q I used the distance from P to R, can someone explain why this worked?
6. Nevermind haha, for some reason I was using the distance on the line PR rather than using the line PQ, thanks!
7. Also, why is there a reaction force at the hinge, but not the fixed point?
8. (Original post by moz4rt)
Nevermind haha, for some reason I was using the distance on the line PR rather than using the line PQ, thanks!
Haha yeah I do that kind of things loads in mechanics, quite often I don't read the questions fully and try and answer an entirely different question
9. (Original post by moz4rt)
I got the correct answer this time, but instead of using the distance from P to Q I used the distance from P to R, can someone explain why this worked?
Was the answer I said correct?
10. (Original post by Nagromicous)
Was the answer I said correct?
No, it was 3g/2, what did you do?
11. (Original post by moz4rt)
Also, why is there a reaction force at the hinge, but not the fixed point?
When you are considering moments about P, you are purely considering the forces on PQ, so in this consideration of moments it doesn't factor it.

But there is a reaction force at R, it has the same magnitude as the tension and acts in the opposite direction, as otherwise the system wouldn't be in equilibrium.
12. (Original post by moz4rt)
No, it was 3g/2, what did you do?
I took moments about P but erroneously used the value for sin(a) when I meant to use the value for cos(a). I wrote down cos(a) but copied the wrong number. :/

5g x (3/2) x cos(a) = 3T
4.5g = 3T
T = 1.5g

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