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# Mechanics-Moments watch

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1. Question 3.

It appears the sole reason I've got an incorrect solution is because I made an incorrect modelling assumption error, and thus treated one moment as a clockwise moment when it should have been an anticlockwise moment.

My question is: How do I deduce which side of the centre of the plank the mass 120N is on?
Had I known this, I would have ended up with an equation I could use to solve for X correctly.
2. (Original post by Illidan2)
Question 3.

It appears the sole reason I've got an incorrect solution is because I made an incorrect modelling assumption error, and thus treated one moment as a clockwise moment when it should have been an anticlockwise moment.

My question is: How do I deduce which side of the centre of the plank the mass 120N is on?
Had I known this, I would have ended up with an equation I could use to solve for X correctly.
That's not necessary knowledge. It would be a whole lot simpler from measure from your pivot, but you're measuring it from A. So then, from what you've drawn, the correct equation for moments is the following:

The error you have is that you said is the distance, which doesn't make sense since clearly when measuring from A so is -ve distance hence makes no sense.
3. I don't think you need to know which side of the centre it is on. Just take moments about D. The unknown reaction then disappears and your only unknown variable left is "x".

The moment produced by the downwards force would then be: 20N x (1m + 1.5m - x)

The rest is easy I hope.
4. (Original post by RDKGames)
That's not necessary knowledge. It would be a whole lot simpler from measure from your pivot, but you're measuring it from A. So then, from what you've drawn, the correct equation for moments is the following:

The error you have is that you said is the distance, which doesn't make sense since clearly when measuring from A so is -ve distance hence makes no sense.
I'm not measuring from A- i'm taking moments from the midpoint, which I have called R(which I shouldn't have, as R is the reaction, but it marks the exact location of the midpoint).

In this position, I do believe I still have 2-x, correct?
5. (Original post by Illidan2)
I'm not measuring from A- i'm taking moments from the midpoint, which I have called R(which I shouldn't have, as R is the reaction, but it marks the exact location of the midpoint).
You are measuring from A, but taking moments about the midpoint.
6. (Original post by RDKGames)
You are measuring from A, but taking moments about the midpoint.
Ahh. So then I WOULD be multiplying 200 by x-2, as from my sketch, x crosses the midpoint, and the distance from A to the midpoint is 2m.

Edit:Ignore that. What I just said makes no sense, and i'm confusing myself now.
7. (Original post by Illidan2)
Ahh. So then I WOULD be multiplying 200 by x-2, as from my sketch, x crosses the midpoint, and the distance from A to the midpoint is 2m.

Ignore that. What I just said makes no sense, and i'm confusing myself now.
Look, we are taking moments about the midpoint. We easily know we have the distance from this point as 2m for both kids. What about the distance from of the CoM from it?

Well, we know that from A it is . Since you labelled it to be on the right side of the pivot, then this distance MUST be greater than 2. Hence, we know that where is the distance from the pivot. And so your distance from the pivot is .
8. Yeah. I see where I went wrong, now, I think
(Original post by RDKGames)
That's not necessary knowledge. It would be a whole lot simpler from measure from your pivot, but you're measuring it from A. So then, from what you've drawn, the correct equation for moments is the following:

The error you have is that you said is the distance, which doesn't make sense since clearly when measuring from A so is -ve distance hence makes no sense.
9. (Original post by Illidan2)
Yeah. I see where I went wrong, now, I think
Yes, I see! As you said in your first post on this thread, it all came down to the 2-x being inconsistent with what I have drawn. Now that you've pointed it out, i've solved for x, and therefore the CoM from A, to be 1/3m. Thank you
10. (Original post by Illidan2)
Yeah. I see where I went wrong, now, I think
Cool. Note that your diagram isn't 'wrong' it's just not the best to work with.

As long as you draw one that's correct at the very least, the rest is just formulation of equations from your diagram.

Someone who might draw the CoM to the left of the pivot and measure it from the pivot would have different eq. to you, namely which would still agree with your answer, but be measured from the pivot rather than A.
11. (Original post by RDKGames)
Cool. Note that your diagram isn't 'wrong' it's just not the best to work with.

As long as you draw one that's correct at the very least, the rest is just formulation of equations from your diagram.

Someone who might draw the CoM to the left of the pivot and measure it from the pivot would have different eq. to you, namely which would still agree with your answer, but be measured from the pivot rather than A.
Awesome. I'm hoping that with time, i'll be able to draw at least marginally better diagrams to work with.
12. (Original post by Illidan2)
Awesome. I'm hoping that with time, i'll be able to draw at least marginally better diagrams to work with.
(Original post by RDKGames)
Cool. Note that your diagram isn't 'wrong' it's just not the best to work with.

As long as you draw one that's correct at the very least, the rest is just formulation of equations from your diagram.

Someone who might draw the CoM to the left of the pivot and measure it from the pivot would have different eq. to you, namely which would still agree with your answer, but be measured from the pivot rather than A.
You very often won't know which side of the middle the unknown force is acting. With this question you do however. With the pivot at the middle and 300N one end and 200N the other, the weight of the plank must be on the same side of the middle as the smaller force if the plank is to stay in equilibrium.

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