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Moments - What stupid mistake am I making ?

I have calculated the mass of the rock is zero, as taking moments about C:

Screenshot 2021-01-20 at 15.57.57.png

With anticlockwise being positive,

60 - 60 - 2W = 0

So 2W = 0
So W = 0.

This obviously is incorrect. But where is my mistake ?

Thanks.
Original post by seals2001
I have calculated the mass of the rock is zero, as taking moments about C:

Screenshot 2021-01-20 at 15.57.57.png

With anticlockwise being positive,

60 - 60 - 2W = 0

So 2W = 0
So W = 0.

This obviously is incorrect. But where is my mistake ?

Thanks.


About what point have you taken moments, and what do each of your terms/numbers represent?
Reply 2
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seals2001
OP
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Original post by ghostwalker
About what point have you taken moments, and what do each of your terms/numbers represent?

Hi there, I took moments about C and said that anticlockwise is positive.

(80 * 0.75) - (0.5 * 120) - 2W = 0

So W = 0.
Original post by ghostwalker
About what point have you taken moments, and what do each of your terms/numbers represent?

The OP states they are taking moments about C for some reason ...
Reply 4
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seals2001
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Original post by Muttley79
The OP states they are taking moments about C for some reason ...

Hi, why is this wrong ? The plank is at equilibrium.
What does 'on the point of tipping about D' mean?
The problem is that this question is about limiting equilibrium. What does that mean?
(edited 3 years ago)
Reply 7
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seals2001
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Original post by Muttley79
What does 'on the point of tipping about D' mean?

I am not too sure about this actually. I have looked it up but cant find any videos on it.
Reply 8
Avatar for Beobobo
Beobobo
8
As the plank is on the point of tilting about D, are the reaction forces staying the same?
(edited 3 years ago)
Original post by seals2001
I am not too sure about this actually. I have looked it up but cant find any videos on it.


when you place something at two points:
image-ef62eebf-5c28-417f-9267-b7e8c205ac996878503111931427346-compressed.jpg.jpeg say the left pen is C and the right one is D and the yellow pen is the plank.
then I tilt the plank about D:
image-fdf4f108-31d4-4794-8d56-006f585494657034127027674220968-compressed.jpg.jpeg
is the plank in contact with the support C? or not? what does that mean?
Original post by Beobobo
As the plank is on the point of tilting about D, I think the reaction at D is no longer 80 N anymore? It should be larger than 80 N as the force of reaction at C is now 0 N. That's why when you take the moment at C using 80 N as the force of reaction at D, the result is wrong. Anyone please correct me if I'm wrong.


You're correct, just don't give too much away right now :smile: let OP think
Reply 11
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seals2001
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Original post by ilovephysmath
The problem is that this question is about limiting equilibrium. What does that mean?

I am not sure,

Original post by ilovephysmath
when you place something at two points:
image-ef62eebf-5c28-417f-9267-b7e8c205ac996878503111931427346-compressed.jpg.jpeg say the left pen is C and the right one is D and the yellow pen is the plank.
then I tilt the plank about D:
image-fdf4f108-31d4-4794-8d56-006f585494657034127027674220968-compressed.jpg.jpeg
is the plank in contact with the support C? or not? what does that mean?

Would the contact force at C be equal to 0 ? As the plank is not in contact with it anymore ? Is that what the plank is on the "point of tilting" at C means; that it is an infinitesimally small distance away from the plank , such that it loses the contact it had with it at that point ?
Reply 12
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Beobobo
8
Original post by ilovephysmath
You're correct, just don't give too much away right now :smile: let OP think

Thank you! :smile: I've edited my post
Original post by seals2001
Would the contact force at C be equal to 0 ? As the plank is not in contact with it anymore ? Is that what the plank is on the "point of tilting" at C means; that it is an infinitesimally small distance away from the plank , such that it loses the contact it had with it at that point ?


Bingo :smile:
you cant take moments at c. the plank is at point of tilting
Reply 15
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seals2001
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Original post by ilovephysmath
Bingo :smile:

That seems like an incredibly specific scenario that is not actually possible to achieve. Am I right in saying this ?

Btw you gave an amazing explanation, thank you !
Original post by seals2001
That seems like an incredibly specific scenario that is not actually possible to achieve. Am I right in saying this ?

Btw you gave an amazing explanation, thank you !


I'm assuming OP's question is year 12 mechanics. we make a lot of assumptions at this stage that are nowhere near reality for example we assume air resistance to be zero or negligible in physics while that's not actually the case unless there's perfect vacuum :smile: but we also need to be aware of what assumptions the spec wants us to make because in reality we might be correct but we won't get the mark :smile: so detailed study is key

also OP, try taking moments about D or A. Your call, try either.
Reply 17
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seals2001
OP
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Original post by ilovephysmath
I'm assuming OP's question is year 12 mechanics. we make a lot of assumptions at this stage that are nowhere near reality for example we assume air resistance to be zero or negligible in physics while that's not actually the case unless there's perfect vacuum :smile: but we also need to be aware of what assumptions the spec wants us to make because in reality we might be correct but we won't get the mark :smile: so detailed study is key

also OP, try taking moments about D or A. Your call, try either.

I got the right answer, thank you :smile:
Original post by seals2001
I got the right answer, thank you :smile:


you're welcome :smile:
Original post by seals2001
I got the right answer, thank you :smile:


Also I just realized you're the OP lmao 😂

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