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resultant force

Two forces, P and Q, are such that the sum of their magnitudes is 45 N. The
resultant of P and Q is perpendicular to P and has a magnitude of 15 N.
Calculate:
(i) the magnitude of P and Q,
(ii) the angle between P and Q.

I get to the point where I have 3 equations:
1) P + Q = 45
2) Qsinx = 15
3) Qcosx - P = 0

I don't know how to resolve from here. Anyone able to help.

Thanks
Reply 1
Original post by bl64
Two forces, P and Q, are such that the sum of their magnitudes is 45 N. The
resultant of P and Q is perpendicular to P and has a magnitude of 15 N.
Calculate:
(i) the magnitude of P and Q,
(ii) the angle between P and Q.

I get to the point where I have 3 equations:
1) P + Q = 45
2) Qsinx = 15
3) Qcosx - P = 0

I don't know how to resolve from here. Anyone able to help.

Thanks

Try using Pythagoras instead of SOHCAHTOA.
(edited 5 years ago)
Original post by bl64
Two forces, P and Q, are such that the sum of their magnitudes is 45 N. The
resultant of P and Q is perpendicular to P and has a magnitude of 15 N.
Calculate:
(i) the magnitude of P and Q,
(ii) the angle between P and Q.

I get to the point where I have 3 equations:
1) P + Q = 45
2) Qsinx = 15
3) Qcosx - P = 0

I don't know how to resolve from here. Anyone able to help.

Thanks


3rd eq gives Qcosx=PQ \cos x = P. Now you can use this and the second equation to say that Q2sin2x+Q2cos2x=152+P2Q^2 \sin^2 x + Q^2 \cos^2 x = 15^2 + P^2

Can you move from there?? This eq has an obvious identity you can use.
Reply 3
Original post by Notnek
Try using Pythagoras instead of SOHCAHTOA.


I only have one value though so how can I do that?
Reply 4
Thanks, can't believe I missed that!
Original post by RDKGames
3rd eq gives Qcosx=PQ \cos x = P. Now you can use this and the second equation to say that Q2sin2x+Q2cos2x=152+P2Q^2 \sin^2 x + Q^2 \cos^2 x = 15^2 + P^2

Can you move from there?? This eq has an obvious identity you can use.
Original post by bl64
Thanks, can't believe I missed that!


So you're good to find the values now? :smile:
Reply 6
Original post by bl64
I only have one value though so how can I do that?


I see actually.

Thanks
Reply 7
Original post by RDKGames
So you're good to find the values now? :smile:


yeh, thanks
Reply 8
Q sinx = 15
Q cosx = P
Try dividing one by the other.
(edited 5 years ago)
Reply 9
Original post by bl64
I only have one value though so how can I do that?

You could have got 152+P2=Q215^2+P^2 = Q^2 by considering Pythagoras instead.

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