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M2 - Statics

Could someone help me with the following question:

Maths problem.png

I tried to work out the distance from P to the 3N force using the sine rule and cosine rule and they both give slightly difference answers?

EDIT: Hmm, in the Edexcel book this question has 4.5m in place of the 5m side. whereas the attached question is from their online solution bank. So maybe it's a dodgy question...

(edited 9 years ago)

Reply 1

ghostwalker

Original post by Sir_Malc

Could someone help me with the following question:

Maths problem.png

I tried to work out the distance from P to the 3N force using the sine rule and cosine rule and they both give slightly difference answers?

Unfortunately there's not a lot you can do with this as the information given in the diagram is not consistent. If you consider the 1N,2N combination, then their point of action is at a vertical height above P of 5sin58, and 6sin40, but those two formulae give different values - :eek:

Reply 2

Sir_Malc

Thanks for the help Ghost.

I edited my original post as that attached question is from the Edexcel solution Bank, yet the Edexcel book has a side of 4.5m in place of the 5m shown above.

That set alarm bells ringing that it may be a dodgy question, which I'm finding is the case with a lot of these M2 edexcel questions.

I presume my method would have been right though, to use either the sin or cosine rule to find the bottom length of the triangle which gives you the perpendicular distance to the 3N force?

Reply 3

ghostwalker

Original post by Sir_Malc

Thanks for the help Ghost.

I edited my original post as that attached question is from the Edexcel solution Bank, yet the Edexcel book has a side of 4.5m in place of the 5m shown above.

The error is certainly reduced by using 4.5, but it's still not exact - about 1% error.

I presume my method would have been right though, to use either the sin or cosine rule to find the bottom length of the triangle which gives you the perpendicular distance to the 3N force?

Yep, by whatever means, I'd go for 6cos40-4.5cos58 - from dropping a vertical from the 1N,2N combination.

(edited 9 years ago)

Reply 4

username1226161

Original post by Sir_Malc

I think you're right, I've spotted many mistakes/discrepancies between the answers in the book and the Solutions Bank, sometimes even within the actual question...