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M1 Mechanics - Forces at an angle (Edexcel)

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Reply 20

ghostwalker

Original post by KiddingMe

Would it be cos20 for this one then?

cos 20 or sin70; either.

Btw, is it better to have angles positive in these types of questions, to avoid errors when calculating?

I find it reduces errors if you make the angle between 0 and 90, sin/cos/tan are then all positive.

Reply 21

KiddingMe

True, using CAST helps. Btw I changed the numbers in front of cos, but still not getting a diff answer.

Reply 22

ghostwalker

Original post by KiddingMe

True, using CAST helps. Btw I changed the numbers in front of cos, but still not getting a diff answer.

A bit cryptic.

What do you currently get?

Reply 23

KiddingMe

I got this:

I've a feeling, I messed up somewhere!

Reply 24

ghostwalker

Original post by KiddingMe

I got this:

I've a feeling, I messed up somewhere!

Your 7.4N forces seems to have changed to 7N.

And for the other two forces you want sin75 and sin63

Reply 25

KiddingMe

Just saw that 7! Getting sloppy here. Btw, why them 2 angles, wouldn't the ones I gave work (I guess not)!

Posted from TSR Mobile

Reply 26

ghostwalker

Original post by KiddingMe

Just saw that 7! Getting sloppy here. Btw, why them 2 angles, wouldn't the ones I gave work (I guess not)!

Posted from TSR Mobile

Draw a little triangle.

With the first force (at A) you had sin70 (or cos 20). Same here. It's the sine of the angle the force makes with the line connecting it to the pivot point.

Reply 27

KiddingMe

Sorry, I'm just confused now haha, could you draw your triangle that u used for one of the forces?

Edit: the sin75 and sin63 you gave, are the same as supplied in the actual question. Is this a coincidence, or simply we should've used them instead of all this complicated stuff!

(edited 11 years ago)

Reply 28

ghostwalker

Original post by KiddingMe

Sorry, I'm just confused now haha, could you draw your triangle that u used for one of the forces?

For the 2.9N force. Component perpendicular to the line from the pivot to the force.

Untitled2.jpg53.6KB

Reply 29

KiddingMe

Sorry Im just seeing what I just did v.wrong .... The 9.3N and 2.9N force should be resolved vertically while the 7.4N should be resolved only horizontally..right?

Reply 30

ghostwalker

Original post by KiddingMe

Sorry Im just seeing what I just did v.wrong .... The 9.3N and 2.9N force should be resolved vertically while the 7.4N should be resolved only horizontally..right?

Yep. The direction perpendicular to the line connecting the force to the pivot.

Reply 31

KiddingMe

Cool! I just drew out the triangles and it came out of the wash as -9.99Nm, which is same as the answer at the back!

Makes more sense now! Cheers unreservedly for all the help!

Btw, do you know any good revision material for these type of questions?

Reply 32

ghostwalker

Original post by KiddingMe

Cool! I just drew out the triangles and it came out of the wash as -9.99Nm, which is same as the answer at the back!

Makes more sense now! Cheers unreservedly for all the help!

Btw, do you know any good revision material for these type of questions?