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# M2) Moments Help!!! watch

1. Hi,

I remember doing this type of question for GCSE additional maths but I completely forgot how to do it.

1. A uniform rod AB of length 12a and weight 4W is suspended by strings attached at C and D, where AC=3a and BD=4a. The breaking strain of the string at C is 3W and that at D is 3.8W. An object of weight W is attached to the beam at a distance x from A. Find the range of values of x if neither string is to break.

I've drawin the correct diagram I think. I've put 3W and 3.8W as T that acts upwards and the 4Wg as downward force.

I tried balancing the forces together so like:

(Wgx)+(4Wg x 6a)=(3W x 3a) + (3.8W + 8a)

and then I tried to solve x. But I'm getting the wrong answer?
and how would I obtain 2 values of x to make a range?

2. (Original post by liemluji)
Hi,

I remember doing this type of question for GCSE additional maths but I completely forgot how to do it.

1. A uniform rod AB of length 12a and weight 4W is suspended by strings attached at C and D, where AC=3a and BD=4a. The breaking strain of the string at C is 3W and that at D is 3.8W. An object of weight W is attached to the beam at a distance x from A. Find the range of values of x if neither string is to break.

I've drawin the correct diagram I think. I've put 3W and 3.8W as T that acts upwards and the 4Wg as downward force.

I tried balancing the forces together so like:

(Wgx)+(4Wg x 6a)=(3W x 3a) + (3.8W + 8a)

and then I tried to solve x. But I'm getting the wrong answer?
and how would I obtain 2 values of x to make a range?

Taking moments about C will give you an equation involving Td but not Tc. Get Td on its own from this equation and then say that Td < 3.8W. Solve that as an inequality for x.

Repeat by taking moments about D.
3. (Original post by liemluji)
Hi,

I remember doing this type of question for GCSE additional maths but I completely forgot how to do it.

1. A uniform rod AB of length 12a and weight 4W is suspended by strings attached at C and D, where AC=3a and BD=4a. The breaking strain of the string at C is 3W and that at D is 3.8W. An object of weight W is attached to the beam at a distance x from A. Find the range of values of x if neither string is to break.

I've drawin the correct diagram I think. I've put 3W and 3.8W as T that acts upwards and the 4Wg as downward force.

I tried balancing the forces together so like:

(Wgx)+(4Wg x 6a)=(3W x 3a) + (3.8W + 8a)

and then I tried to solve x. But I'm getting the wrong answer?
and how would I obtain 2 values of x to make a range?

4. (Original post by tiny hobbit)

Taking moments about C will give you an equation involving Td but not Tc. Get Td on its own from this equation and then say that Td < 3.8W. Solve that as an inequality for x.

Repeat by taking moments about D.
So we don't take the added weight W into calculating moment about C?

(Td x 5a) - (4Wg x 3a) < 3.8W
or
(Td x 5a)- (4Wg x 3a) - (Wg x 'x' <3.8W?

I still cant get the answer. Do you mind helping me more :'(

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