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# M1 Mechanics - Moments? watch

1. Hi.

I was trying out some questions on Moments, and I just cannot hack this one in particular!

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2. (Original post by FaisalTreShah)
Hi.

I was trying out some questions on Moments, and I just cannot hack this one in particular!

What have you tried/what are your thoughts?
3. (Original post by usycool1)
What have you tried/what are your thoughts?
Well so far, I've just ignored the particle of weight and I've calculated the reaction forces, where C = 100N and D = 120N. From here I just have no idea on what I should do. D:
4. (Original post by FaisalTreShah)
Well so far, I've just ignored the particle of weight and I've calculated the reaction forces, where C = 100N and D = 120N. From here I just have no idea on what I should do. D:
OK, there wasn't really any need to do that.

I suggest you first draw your own diagram with a point "E" marked on it for the particle to make things clearer.

Then resolve vertically to get the magnitude of the reactions at C and D in terms of W.

Make sense? See if you can go on from there.
5. (Original post by FaisalTreShah)
Well so far, I've just ignored the particle of weight and I've calculated the reaction forces, where C = 100N and D = 120N. From here I just have no idea on what I should do. D:
Resolve at specific points of the rod?
6. (Original post by LaserBeamedOrgs)
Resolve at specific points of the rod?
No, you resolve all the forces acting vertically on the rod. It is in equilibrium so there cannot be a net force. You need not worry about their point of application because in equilibrium there will be no unbalanced moment, either.
7. (Original post by usycool1)
OK, there wasn't really any need to do that.

I suggest you first draw your own diagram with a point "E" marked on it for the particle to make things clearer.

Then resolve vertically to get the magnitude of the reactions at C and D in terms of W.

Make sense? See if you can go on from there.
Thank you so much! I've just tried doing it your way, but this is what I've managed to end up with... somehow. o_O

I know your method is right but I know I've gone wrong somewhere. Can you please show me my error?
Attached Images

8. (Original post by FaisalTreShah)
Thank you so much! I've just tried doing it your way, but this is what I've managed to end up with... somehow. o_O

I know your method is right but I know I've gone wrong somewhere. Can you please show me my error?
You've taken Moments at A and B but haven't included that both RC and RD are upward forces.
Moments at A: 2.5RD + RC = Wx + 2*150
Moments at B: 1.5RD + 3RC = Wx + 2*150
It should all come out from here.
9. (Original post by Brister)
No, you resolve all the forces acting vertically on the rod. It is in equilibrium so there cannot be a net force. You need not worry about their point of application because in equilibrium there will be no unbalanced moment, either.
That is what you initially do, then you resolve around a specific point, refer to the latest post on the thread, those equations that he has displayed, those can only be achieved by resolving clockwise and anticlockwise at a point on the rod
10. (Original post by LaserBeamedOrgs)
That is what you initially do, then you resolve around a specific point, refer to the latest post on the thread, those equations that he has displayed, those can only be achieved by resolving clockwise and anticlockwise at a point on the rod
Yes, we call that taking moments. You resolve forces and take moments. I just misunderstood your statement.
11. I take it you now understand what you have to do, just remember to always take moments about the same point
12. (Original post by Season One)
I take it you now understand what you have to do, just remember to always take moments about the same point
Again, thank you everyone who took their time out to help me. Unfortunately, I've STILL got it wrong (by this point half the internet world screams in frustration and agony to my lack of intelligence regarding Moments ¬_¬) but it's most likely due to how I've approached the question.

It makes no sense! Usually I'm good at this topic, but this question alone has made me change my mind. -_-

I will keep practising, I may also ask my teacher for help. Thank you all once more for helping me out.
13. (Original post by FaisalTreShah)
Again, thank you everyone who took their time out to help me. Unfortunately, I've STILL got it wrong (by this point half the internet world screams in frustration and agony to my lack of intelligence regarding Moments ¬_¬) but it's most likely due to how I've approached the question.

It makes no sense! Usually I'm good at this topic, but this question alone has made me change my mind. -_-

I will keep practising, I may also ask my teacher for help. Thank you all once more for helping me out.
Yeah asking your teacher would probably be the best idea, hard to explain over the Internet

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