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# M4 Potential Energy Q watch

1. Hi, I’m having some trouble with this q, help would be greatly appreciated. I don’t understand how to calculate the GPE of the two masses of 3mg/2.

Question is from January 2006.
2. (Original post by k92e67)
I don’t understand how to calculate the GPE of the two masses of 3mg/2.
Think of R being at B say.

As it slides down, RB gets longer and the mass on the right rises by the same amount.

Similarly as RA gets shorter, the mass on the left drops by the same amount.

So you're interested in the lengths of RB and RA.
3. (Original post by ghostwalker)
Think of R being at B say.

As it slides down, RB gets longer and the mass on the right rises by the same amount.

Similarly as RA gets shorter, the mass on the left drops by the same amount.

So you're interested in the lengths of RB and RA.
This is where I got to. So as R slides down, the increase in RB is equal to the decrease in the length of the right mass which is also the same as the increase in the length of the left hand side mass.

I realise the lengths of RB and RA are of interest, but its figuring out how to incorporate them into an equation for gpe that im finding difficult. Not sure what im missing
4. (Original post by k92e67)
This is where I got to. So as R slides down, the increase in RB is equal to the decrease in the length of the right mass which is also the same as the increase in the length of the left hand side mass.

I realise the lengths of RB and RA are of interest, but its figuring out how to incorporate them into an equation for gpe that im finding difficult. Not sure what im missing
I am also struggling with this question. However, can you use the fact that OB and OR are both radii and therefore equal to a which makes triangle BOR an isosceles triangle?
5. (Original post by k92e67)
This is where I got to. So as R slides down, the increase in RB is equal to the decrease in the length of the right mass which is also
True. But the phrase "decrease in the lenght of the right mass" is confusing at best. Better to say the increase in height of the right mass.

the same as the increase in the length of the left hand side mass.
This isn't true. The change in heights of the right and left masses will generally be different.

Lets look at the right mass. The length of the string from R via B to the mass is a constant.

So, the height of the right mass is "length of string" minus RB, relative to the lowest point it can be

So, GPE is (3mg/2)("a constant" - RB)

And do something similar for the left.

Edit: Actually that was the depth of the right mass. Minus that to get it's height above the lowest point.
Since you've solved this though, I won't do a separate post.
6. (Original post by ghostwalker)
True. But the phrase "decrease in the lenght of the right mass" is confusing at best. Better to say the increase in height of the right mass.

This isn't true. The change in heights of the right and left masses will generally be different.

Lets look at the right mass. The length of the string from R via B to the mass is a constant.

So, the height of the right mass is "length of string" minus RB, relative to the lowest point it can be

So, GPE is (3mg/2)("a constant" - RB)

And do something similar for the left.
thank you very much. I just attempted it again and got it right. was stuck on this for ages..

(Original post by Pepperpeople)
I am also struggling with this question. However, can you use the fact that OB and OR are both radii and therefore equal to a which makes triangle BOR an isosceles triangle?
yes thats the approach i used. i dont think there is another way anyway.

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