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###### A Level Maths Moments question

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2 weeks ago

Hi can someone help me with this question? I got M = 0.77 but apparently it's wrong

A uniform rod AB rests horizontally on two smooth pivots at the points A and C. The rod is of length 5m and mass 12kg. The distance BC is 1.5m. A mass of Mkg is attached to the rod at B so that the rod remains horizontal and in equilibrium. The reaction at the pivot at C is three times the reaction at the pivot at A. By taking moments around B, it can be shown that the reaction at A is 31.58N. Calculate the value of M.

A uniform rod AB rests horizontally on two smooth pivots at the points A and C. The rod is of length 5m and mass 12kg. The distance BC is 1.5m. A mass of Mkg is attached to the rod at B so that the rod remains horizontal and in equilibrium. The reaction at the pivot at C is three times the reaction at the pivot at A. By taking moments around B, it can be shown that the reaction at A is 31.58N. Calculate the value of M.

(edited 2 weeks ago)

Original post by 888ella

Hi can someone help me with this question? I got M = 0.77 but apparently it's wrong

A uniform rod AB rests horizontally on two smooth pivots at the points A and C. The rod is of length 5m and mass 12kg. The distance BC is 1.5m. A mass of Mkg is attached to the rod at B so that the rod remains horizontal and in equilibrium. The reaction at the pivot at C is three times the reaction at the pivot at A. By taking moments around B, it can be shown that the reaction at A is 31.58N. Calculate the value of M.

A uniform rod AB rests horizontally on two smooth pivots at the points A and C. The rod is of length 5m and mass 12kg. The distance BC is 1.5m. A mass of Mkg is attached to the rod at B so that the rod remains horizontal and in equilibrium. The reaction at the pivot at C is three times the reaction at the pivot at A. By taking moments around B, it can be shown that the reaction at A is 31.58N. Calculate the value of M.

What did you do?

Original post by mqb2766

What did you do?

I did (94.74 x 3.5) = (12g x 2.5) + (5 x Mg) and then rearranged to find M = 0.7671...

Reply 3

2 weeks ago

Original post by 888ella

I did (94.74 x 3.5) = (12g x 2.5) + (5 x Mg) and then rearranged to find M = 0.7671...

Maybe because reaction at A is not 31.58N but actually approx 30.95N (using g=9.8).

Original post by RDKGames

Maybe because reaction at A is not 31.58N but actually approx 30.95N (using g=9.8).

maybe they used 9.81 for g

Reply 5

2 weeks ago

Original post by the bear

maybe they used 9.81 for g

Then the reaction is 30.98N.

Reaction in terms of g should be 60g/19

(edited 2 weeks ago)

Original post by 888ella

I did (94.74 x 3.5) = (12g x 2.5) + (5 x Mg) and then rearranged to find M = 0.7671...

With g=10 you get the stated reaction(s), and slightly simpler than taking moments to get M, you could simply balance vertically so 4R - 12g.

Reply 7

2 weeks ago

Original post by mqb2766

With g=10 you get the stated reaction(s), and slightly simpler than taking moments to get M, you could simply balance vertically so 4R - 12g.

Been so long since I used g=10 I forgot these questions even use that as an option.

Wish whoever wrote the question made this clear since OP used the wrong value of 9.8 by the look of it.

Original post by RDKGames

Been so long since I used g=10 I forgot these questions even use that as an option.

Wish whoever wrote the question made this clear since OP used the wrong value of 9.8 by the look of it.

Wish whoever wrote the question made this clear since OP used the wrong value of 9.8 by the look of it.

It could be a frankenstein question which is adapted from another which did use g=10 or ... It seems strange as well to give R as you get M without taking moments and its obviously a moments question.

Original post by mqb2766

It could be a frankenstein question which is adapted from another which did use g=10 or ... It seems strange as well to give R as you get M without taking moments and its obviously a moments question.

I got a new answer of 0.89 but idk if its right, what did you get?

Original post by 888ella

I got a new answer of 0.89 but idk if its right, what did you get?

As in the previous posts, it really depends on the value of g, and in the question they seem to be using g=10 which gives a bit different value (using weight 4R-12g its easy to calculate). The answer is a bit < 1 kg just based on simple physics. You said in the OP that 0.77 is wrong, do you have the answer/is it an online test?

Original post by mqb2766

As in the previous posts, it really depends on the value of g, and in the question they seem to be using g=10 which gives a bit different value (using weight 4R-12g its easy to calculate). The answer is a bit < 1 kg just based on simple physics. You said in the OP that 0.77 is wrong, do you have the answer/is it an online test?

I got it! Its 0.63 im supposed to use g=10

Original post by 888ella

I got it! Its 0.63 im supposed to use g=10

As mentioned above, it sounds a bit of a strange question and its surprising youre both given R and youre using g=10 without it being clear, unless that is the norm on your course. But youre sorted so ...

Original post by mqb2766

As mentioned above, it sounds a bit of a strange question and its surprising youre both given R and youre using g=10 without it being clear, unless that is the norm on your course. But youre sorted so ...

I think my teacher just took part of a question without giving us the whole thing so that's why it was confusing, not my fault! xD

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