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# M3 Elastic Strings Question Help

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1. Can someone help me answer the following question:

A particle P of mass 2kg is attached to the mid-point of a light elastic string of natural length 2m and modulus of elasticity 20N. One end of the string is attached to point A of a ceiling and the other end is attached to point B on the floor 3m vertically below A. Calculate the tension in AP and the tension in PB.

Thanks
2. (Original post by Student 977)
Can someone help me answer the following question
Sure. What thoughts of your own do you have? What have you already tried? What specific issue are you having?

I understand that from Hooke's law, the total tension in the string is 10N, but I am unsure of where to go from there.
4. (Original post by Student 977)

I understand that from Hooke's law, the total tension in the string is 10N, but I am unsure of where to go from there.
Why don't you draw a sketch? Label the tensions in AP to be T_1 and the tension in BP to be T2.

Now resolve up and down. You have T_1 - T_2 = 2g.

The net tension in each part of the string, T=10N

For the top half T1-2g=10 (i) and for the bottom T2+2g=10 (ii)

Can you take it from here?
5. Where do equations (i) and (ii) come from?
6. (Original post by Student 977)
Where do equations (i) and (ii) come from?
Like you said, the net tension in each part of the string is 10, so resolve up and down.
7. But why is the weight in (i). The only force in the top part of the string is the tension surely?
8. (Original post by Student 977)
But why is the weight in (i). The only force in the top part of the string is the tension surely?
It has the weight pulling down on the string whilst the tension pulls up on the string, surely?
9. Ok, I got the answer now (19.8 and 0.2). Thanks for your help
10. (Original post by Student 977)
Ok, I got the answer now (19.8 and 0.2). Thanks for your help
No worries.

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