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Mechanics: Resultant forces

The resultant of two forces of magnitude F1 and F2 has magnitude 20N. If F1 = 8N and the angle between the two forces is 70o, find f2.
I am completely stuck and have been trying to work through this for a couple of hours now. Please can someone give me a step-to-step guide on how to complete this question. Many thanks :biggrin:
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TenOfThem
18
Original post by firsedge
The resultant of two forces of magnitude F1 and F2 has magnitude 20N. If F1 = 8N and the angle between the two forces is 70o, find f2.
I am completely stuck and have been trying to work through this for a couple of hours now. Please can someone give me a step-to-step guide on how to complete this question. Many thanks :biggrin:


Draw a diagram
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firsedge
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Original post by TenOfThem
Draw a diagram


I have drawn a diagram but it still isn't making any sense
Reply 3
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TenOfThem
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Original post by firsedge
I have drawn a diagram but it still isn't making any sense


Is it a triangle

Does it show F1 followed by F3 with the result being the longest side

Does it have an angle of 110 in it
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firsedge
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Yeah it is a triangle, with that. I just don't get how to interpret the figures to get F2
Reply 5
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TenOfThem
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Original post by firsedge
Yeah it is a triangle, with that. I just don't get how to interpret the figures to get F2


cosine rule
The cosine rule is about right.

Try to figure the following from your diagram:

202 = 82 + (F2 ) 2 - 2(8) (F2) cos 70o

Hope this helps. Peace.
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firsedge
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Original post by WhiteGroupMaths
The cosine rule is about right.

Try to figure the following from your diagram:

202 = 82 + (F2 ) 2 - 2(8) (F2) cos 70o

Hope this helps. Peace.


Hi Thank you for that! I think I must of misread to question wrong last night (even when I typed it onto here) as I got F1 as being 80N not 8.
I have the quadratic with 21.2.. and -15.79... How do I know which one is the right answer?

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