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Edexcel m2 helppppp centre of mass

Hi. I'm so sorry but could someone please show me the light. Mechanics is so sad. The exam is next week. Halp. :frown:

I don't understand where to even begin when it comes to Centre of Mass questions where you've found the centre of mass in part a, but in part b you have to find the value of k when a mass of kM is attached (to the lamina). For example, I am struggling with Edexcel M2 June 2015 q2b. In general what do I do?

http://pmt.physicsandmathstutor.com/download/Maths/A-level/M2/Papers-Edexcel/June%202015%20QP%20-%20M2%20Edexcel.pdf

Thank you so much.
*cries from Mechanics*
Original post by boodledoodle123
Hi. I'm so sorry but could someone please show me the light. Mechanics is so sad. The exam is next week. Halp. :frown:

I don't understand where to even begin when it comes to Centre of Mass questions where you've found the centre of mass in part a, but in part b you have to find the value of k when a mass of kM is attached (to the lamina). For example, I am struggling with Edexcel M2 June 2015 q2b. In general what do I do?

http://pmt.physicsandmathstutor.com/download/Maths/A-level/M2/Papers-Edexcel/June%202015%20QP%20-%20M2%20Edexcel.pdf

Thank you so much.
*cries from Mechanics*


When the mass kMkM is added onto the lamina, the overall centre of mass moves. We are told that when the system S is freely suspended from A, the line OA is the vertical. Now you need to know that the centre of mass of S must be on the vertical through, so it must be on the line through O and A.

This means you have this situation as shown below. The blue force lines are parallel to the green line OA, as mass acts parallel to it when hung in equilibrium. As the CoM is on the green line, and equilibrium is obtained, this means clockwise and anticlockwise moments on either side of the green line must cancel each other out. The two moments here in question are the forces from the two centres of mass.

Does this make more sense, and can you see how you can finish off the question by considering moments?

(edited 6 years ago)
Original post by RDKGames
When the mass kMkM is added onto the lamina, the overall centre of mass moves. We are told that when the system S is freely suspended from A, the line OA is the vertical. Now you need to know that the centre of mass of S must be on the vertical through, so it must be on the line through O and A.

This means you have this situation as shown below. The blue force lines are parallel to the green line OA, as mass acts parallel to it when hung in equilibrium. As the CoM is on the green line, and equilibrium is obtained, this means clockwise and anticlockwise moments on either side of the green line must cancel each other out. The two moments here in question are the forces from the two centres of mass.

Does this make more sense, and can you see how you can finish off the question by considering moments?


I've just taken moments and it works!
The worded explanation was very useful - thank you :smile:

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