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# A Level Physics Moments watch

1. Hi TSR,

I don’t understand for this question why we have to use moments to answer the question correctly. Surely, we can use equating forces(upward forces = downward forces) instead of using moments? Can someone please explain to me where the flaw in my logic is please.

Btw I don’t need the answer I am just a bit stuck on my logic that’s all.

Thanks
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2. I think you have to calculate the centre of gravity of the sign, and calculate the vertical component of the metal rod, and taking moments at the hinge.

For a system to be in equilibrium, the net force = 0 and the moments = 0, so it is not accelerating or rotating.

In theory, I believe you are equating forces, the weight is downwards and the tension is upwards.
3. (Original post by Guarddyyy)
I think you have to calculate the centre of gravity of the sign, and calculate the vertical component of the metal rod, and taking moments at the hinge.

For a system to be in equilibrium, the net force = 0 and the moments = 0, so it is not accelerating or rotating.

In theory, I believe you are equating forces, the weight is downwards and the tension is upwards.
So how do I know when to equate forces and when to take moments?
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4. What's the correct answer? I got 588.6N. If it's right I'll show you how I did it. My physics isn't the best tho lol so not sure if it is.
5. (Original post by ahussainxo)
What's the correct answer? I got 588.6N. If it's right I'll show you how I did it. My physics isn't the best tho lol so not sure if it is.
The correct answer is 290N. You did it but equating forced directly. But in fact you have to take moments for some bizarre reason instead. Hence my question when do you take moments and when do you just equate forces?
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6. (Original post by Yodalam)
The correct answer is 290N. You did it but equating forced directly. But in fact you have to take moments for some bizarre reason instead. Hence my question when do you take moments and when do you just equate forces?
Oh riiight. I've just calculated again and got 294N.
7. I got the answer of 294.3N which is about 290N. Personally, I think that if you keep practising these questions you'd be able to spot when to use moments and when to equate forces. There is a pivot which I would assume hints at using moments since it can move at that point.

All moments questions pretty much look the same anyway
8. Is there a scientific explanation? Is it linked to equilibrium or something?
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9. I would say it's linked to equilibrium. Since you have a pivot, there's moments involved, as well as the weight. Equilibrium with a pivot means moments = 0 and net force = 0, so nothing is moving.

I hope that clears it up, otherwise, I'm out of explanations, unfortunately :C
10. wait are you sure the answer is 290N? because all you did was 30x9.81? wuuuuttt, what is the 80cm for?
11. (Original post by ibyghee)
wait are you sure the answer is 290N? because all you did was 30x9.81? wuuuuttt, what is the 80cm for?
The answer is 294N rounded to (2sf)
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12. (Original post by Yodalam)
The answer is 294N rounded to (2sf)
that 3sf?
13. (Original post by ibyghee)
wait are you sure the answer is 290N? because all you did was 30x9.81? wuuuuttt, what is the 80cm for?
If you do the method of using moments and calculate the centre of gravity of the sign then calculate the clockwise and anticlockwise moments which equal 0.

So you have 0.4 * 30 * 9.81 = T * sin30 * 0.8, which happens to be the same as 30 * 9.81, Unless I'm missing something
14. (Original post by ibyghee)
that 3sf?
Sorry my previous answer of 290N was 2sf. The non rounded answer is 294N. Sorry for the confusion
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15. (Original post by Guarddyyy)
If you do the method of using moments and calculate the centre of gravity of the sign then calculate the clockwise and anticlockwise moments which equal 0.

So you have 0.4 * 30 * 9.81 = T * sin30 * 0.8, which happens to be the same as 30 * 9.81, Unless I'm missing something
Ye hmmm weird, i think because 80 cm is so close to 1m it makes little difference.
16. This is how I did it.
Attached Images

17. I hope the theory behind it makes sense, if not I could try my best to make it clearer in any parts you don't understand. Then again, I'm just a year 13 student trying to get the A* in physics. The only thing dragging me down is stupid explanations.
18. (Original post by Yodalam)
The correct answer is 290N. You did it but equating forced directly. But in fact you have to take moments for some bizarre reason instead. Hence my question when do you take moments and when do you just equate forces?
You have to take moments in this example as the forces are all acting around a pivot so you can't just equate forces.
19. (Original post by Yodalam)
The correct answer is 290N. You did it but equating forced directly. But in fact you have to take moments for some bizarre reason instead. Hence my question when do you take moments and when do you just equate forces?
You have to take moments about the hinge because that eliminates the forces acting there.
20. Hey so to answer your question, you use moments here as it's not in equilibrium. You resolve forces when an object is in equilirbium. Here, the object moves as it's on a hinge, so it's not in equilibrium. Therefore in this instance you'd use moments. Sorry for the late response but I hope it helps.

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Updated: October 19, 2018
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