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[mulligc3]

Can anybody help me with this physics problem please?It's driving me mad.

A 10 kg picture is hanging on a wall by two ropes
Rope 1 makes an angle of 60 degrees with the horizontal and rope 2 makes an angle of 30 degrees.
What is the force acting on rope A?


How would you go about it?

10*9.8= 98 N
F1 cos 60- F2 cos 30=0 as pic isn't moving
F1 sin 60 + F2 sin 30 =98
then substitution but my answer is always wrong.

[UknowsI]

6 years since last time I studied physics, so not quite sure if it's right, but I would do something like this:

sin(30)A=sin(60)B
sin(60)A+sin(30)B=10 Kg

Which gives you A = 8.66 Kg or 85 N

On second look, this is exactly the same as you wrote. It should give you something like A=98/(sin(PI/3)+((sin(PI/6))^2)/sin(PI/3))

[mulligc3]

Thanks but that's not it. I don't know the answer but I know that's not right.
It's question 3c on this website
http://library.thinkquest.org/10796/ch5/ch5q.htm
so you enter the ans and they tell ya if you are right or not.

I'm doing a physics course next year and so trying to revise what I should know!

[UknowsI]

I live by the saying "If at first I don't succeed, it's probably their mistake"

The right answer according to the page is 49 Newton, looks like they have calculated the force on rope B instead.

[mulligc3]

maybe they mixed it up, and you are right!

sin(60)A+sin(30)B=10 Kg this is the y component force up and down

sin(30)A=sin(60)B is this the x component?
why did you use sin, should it not be cos for x unless you are using the internal angles??

[UknowsI]

sin(30)A=sin(60)B are the sideway forces. I think it's exactly the same as F1 cos 60- F2 cos 30=0 as you wrote.

I used sin instead of cos because sinus was listed first when I looked it up on wikipedia. But sin(30)=cos(60) and sin(60)=cos(30), so it doesn't really matter.

As far as I can see you had it right all along. If you calculate F2 in your formulas it should give you 49 N.

[Drew345]

I get that Rope A has 85 N (42.3 in vertical and 73.6 in horizontal)
Rope B has 49 N (42.5 in vertical and 24.5 in horizontal)

It all checks out that the two vertical forces cancel out and the two horizontal forces add up to 98 N

I think you are right and they are wrong. It happens.

[mulligc3]

Thanks, It worked out this morning using them equations.
Amazing what a fresh mind can do.Last night I'd been mixing up signs and getting frustrated so calculations didn't work!!

Thanks a mill

[djsmnc]

I usually solve these problems by saying that the Lord wills the amount of force

[maingman]

http://www.resources-teachers.info/?cat=28

[mulligc3]

gomapsumnida