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M3: COM

can someone explain to me how I am suppose to do b and please explain the logic behind it? thank you

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Bumpings :smile:
hello?
to me it looks like VA makes an 45 degree angle with the horizontal! :frown:

Original post by tiny hobbit
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Original post by Muttley79
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Original post by old_engineer
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Original post by Shaanv
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Original post by RDKGames
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thanks
Original post by Maths&physics
to me it looks like VA makes an 45 degree angle with the horizontal! :frown:


Well... yeah..

If it's a 45 degree angle to the vertical then its a 45 degree angle to the horizontal since the two must sum to 90!
Original post by RDKGames
Well... yeah..

If it's a 45 degree angle to the vertical then its a 45 degree angle to the horizontal since the two must sum to 90!


lol.... do you understand the proceeding steps?
Original post by Maths&physics
lol.... do you understand the proceeding steps?


Yeah, what is it you don't get?
Original post by RDKGames
Yeah, what is it you don't get?


thats ho wi understand VA being at a 45 degree angle to the vertical through V?!! :s-smilie:
Original post by Maths&physics
thats ho wi understand VA being at a 45 degree angle to the vertical through V?!! :s-smilie:


Seems like too big of an angle to be 45 doesn't it?

How about if you extend the vertical all the way down and consider the acute one angle between VA and it. That's what the mark scheme is doing and what all of these types of questions will refer to most of the time when they say 'angle to the vertical'
Original post by RDKGames
Seems like too big of an angle to be 45 doesn't it?

How about if you extend the vertical all the way down and consider the acute one angle between VA and it. That's what the mark scheme is doing and what all of these types of questions will refer to most of the time when they say 'angle to the vertical'


oh yeah, I misunderstood: practise makes perfect.
Original post by Maths&physics
to me it looks like VA makes an 45 degree angle with the horizontal! :frown:
thanks


This is correct - 45 degrees to the horizontal is the same as 45 degrees to the vertical.
Original post by RDKGames
Seems like too big of an angle to be 45 doesn't it?

How about if you extend the vertical all the way down and consider the acute one angle between VA and it. That's what the mark scheme is doing and what all of these types of questions will refer to most of the time when they say 'angle to the vertical'


ok, so I see the angle between V and VA is 45. now, we're taking moments about V do we spin it around in a vertical way, or do we spin around V horizontally, so that B makes a full circle?
Original post by Muttley79
This is correct - 45 degrees to the horizontal is the same as 45 degrees to the vertical.


I forgot when they say the vertical in these questions, it means the vertical passes through the new COM.

can you see the above? thanks
Original post by Maths&physics
ok, so I see the angle between V and VA is 45. now, we're taking moments about V do we spin it around in a vertical way, or do we spin around V horizontally, so that B makes a full circle?


Er... there's no spinning mate.

There are two forces Mg and kMg with a certain perpendicular distance away from the vertical and you need to know what that is.

Taking moments about V with the thing being in equilibrium means that these moments cancel out.
Moment = Force x perp. distance
Original post by Maths&physics
I forgot when they say the vertical in these questions, it means the vertical passes through the new COM.

can you see the above? thanks


Why is RDKGames talking about spinning?

The object is in equilibrium and attached so it is not moving.
Original post by RDKGames
Er... there's no spinning mate.


thats how exam solutions guy taught it in M1. so what does it mean take by taking moments about V - if its not spinning?

There are two forces Mg and kMg with a certain perpendicular distance away from the vertical and you need to know what that is.

Taking moments about V with the thing being in equilibrium means that these moments cancel out.
Moment = Force x perp. distance


is the diagram correct? how do I calculate the perp distances? thanks
Original post by Muttley79
Why is RDKGames talking about spinning?

The object is in equilibrium and attached so it is not moving.


the exam solutions guy used to talk about spinning in either clockwise or anticlockwise when referring to moments in M1. what happens when you take moments about a point - how do you visualise whats happening?
Original post by Maths&physics
the exam solutions guy used to talk about spinning in either clockwise or anticlockwise when referring to moments in M1. what happens when you take moments about a point - how do you visualise whats happening?


Oh I see - I don't teach it like that. For students who struggle with moments I get them to lay a pencil on the force and then calculate the perpendicular distance from the pivot and then it also should be easier to see whether its effect is clockwise or anticlockwise.
Original post by Muttley79
Oh I see - I don't teach it like that. For students who struggle with moments I get them to lay a pencil on the force and then calculate the perpendicular distance from the pivot and then it also should be easier to see whether its effect is clockwise or anticlockwise.


I drew a diagram, attached above. is it the perp force from the vertical or from V (where its attached)? oi guess, the larger perp distance, equates to direction of moment (clockwise, etc)? is this wasnt in eq, would it be spinning and creating a circle around V?
Original post by Maths&physics
thats how exam solutions guy taught it in M1. so what does it mean take by taking moments about V - if its not spinning?



is the diagram correct? how do I calculate the perp distances? thanks


Use the right angled triangle joining your blue line to V and the one joining the green line to V. You can work out the angles in these using the 45 degrees and the angle in the triangle with sides a and 2a.

Use these to work out the lengths of the blue and green lines and then take moments about V.

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