The Student Room Group

Can't seem to figure out what the answer is?

Hi guys, again sorry about the constant posts but the amount of questions I can't do is really bothering me:frown:

The resultant of two forces,x and 20N,is 27.5N at 16 degrees to the 20N force. Find the magnitude of x and the angle it makes with 20N

Rep will obv be given, can you please explain how youcame up with the answer, as I have tried resolving 27.5N to get the vertical (27.5sin16) but I don't think that is right?


Once agn thanks
Reply 1
Just draw the relevant triangle and use trigonometry, that's all you ever need to do. You don't need to resolve horizontally and vertically, just use the sine and cosine rule.

Should look a bit like this (ignore the captions):

Reply 2
Xei
Just draw the relevant triangle and use trigonometry, that's all you ever need to do. You don't need to resolve horizontally and vertically, just use the sine and cosine rule.

Should look a bit like this (ignore the captions):


Thank you so much!!! I think I will just use sine and cosine instead of doing all that resolving rubbish

Rep+! And once agn thanks for the reply to my email also, I will rep u next month also:biggrin:
Reply 3
lol no prob. :smile:

Resolving vertically and horizontally is useful if there's three vectors or more perhaps, but when you only have two, yeah, just use sin/cos rules.
Reply 4
Xei
lol no prob. :smile:

Resolving vertically and horizontally is useful if there's three vectors or more perhaps, but when you only have two, yeah, just use sin/cos rules.

Hey dude, I have a little problem with using the cosine rule... because when I do the cosine rule and find out the length, the resultant is smaller than the other force?
I get the answer to be 9.6 something?

and i dont think thats the correct answer? especially as this is meant to be the resultant force.
Reply 5
You're already given the resultant in the question.

The triangle you draw should have 20 where the diagram says 'current vector', 27.5 where it says 'resultant vector', and the angle between the two should be 16 degrees. The 'second vector' is x.

You can use the cosine rule to find x (I make it 25.05N if you want to check) and then use the sine rule to get the obtuse angle between the 20N vector and vector x; then subtract this from 180 to get the correct angle.