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# Maths - Mechanics - On a string. watch

1. How do I get my equations when given a diagram for example:

You have 2 spherical objects on a light inextensible string (on the peg). Then, the string on one side goes downwards on one side.. How do I get the equations the right way round?

Cos usually one is -T in one equation and the other is usually +T?
2. the one which is going to move down will have a larger mg than T... so put -T
3. (Original post by MKaur18)
How do I get my equations when given a diagram for example:

You have 2 spherical objects on a light inextensible string (on the peg). Then, the string on one side goes downwards on one side.. How do I get the equations the right way round?

Cos usually one is -T in one equation and the other is usually +T?
Can you give an example? Are we talking about connected particles?

We use F=ma on both, one particle is moving down so

Mg - T = Ma because the weight is pulling it down

On the lighter particle the tension is pulling it up

T - mg = ma

Is this what you mean?
4. (Original post by Muttley79)
Can you give an example? Are we talking about connected particles?

We use F=ma on both, one particle is moving down so

Mg - T = Ma because the weight is pulling it down

On the lighter particle the tension is pulling it up

T - mg = ma

Is this what you mean?
Yes. Right ok, thanks. Did you memorise that then apply it to questions? Cos, I did it a while ago, and forgot how to approach those questions.. :O oops.

(Original post by the bear)
the one which is going to move down will have a larger mg than T... so put -T
Ok.
5. (Original post by MKaur18)
Yes. Right ok, thanks. Did you memorise that then apply it to questions? Cos, I did it a while ago, and forgot how to approach those questions.. :O oops.

Ok.
No, I get students to draw a diagram, decide which one is moving down, add an arrow to show that and call acceleration 'a'.

Then explain: using F = ma on particle A gives ....
6. (Original post by Muttley79)
No, I get students to draw a diagram, decide which one is moving down, add an arrow to show that and call acceleration 'a'.

Then explain: using F = ma on particle A gives ....
After drawing the diagrams, I struggle to figure if T is negative.. but I think I've got it.

T is negative on the one going downwards (heavier one).
7. (Original post by MKaur18)
After drawing the diagrams, I struggle to figure if T is negative.. but I think I've got it.

T is negative on the one going downwards (heavier one).
Yes, the weight of the heavier one is pulling the system down and the tension in the string is acting against gravity. A diagram with forces marked on it really helps.

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Updated: February 3, 2018
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