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Physics Problem

  • Thread starter frozen7
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https://www.physicsforums.com/attachment.php?attachmentid=4224&stc=1

The angle between A and horizontal is 60, angle between D and horizontal is 70,angle between BC and horizontal is 30. Mass of P is 2kg.
Find: (a) Tension in AB, BC and CD
(b) Mass of Q.

I have been trying to solve this question by using the following 3 equations but I still cant get the answer: ABsin60 + BCsin30 = 2g
CDsin70 + BCsin30 = m(Q)g
ABcos60 = CDcos70 + BCcos30
Can anyone help me to solve this question? Thanks. :smile:
 

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Doc Al

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frozen7 said:
ABsin60 + BCsin30 = 2g
This one's OK (vertical forces at B).
CDsin70 + BCsin30 = m(Q)g
Vertical forces at C. Problem: Tension "BC" pulls down at C.
ABcos60 = CDcos70 + BCcos30
Where did this one come from? Instead, consider horizontal forces at B and C.
 
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The part I dont understand the most is about the horizontal force....
Is ABcos60 = BC cos30 and BCcos30 = CDcos70 ??
 

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frozen7 said:
Is ABcos60 = BC cos30 and BCcos30 = CDcos70 ??
That looks correct to me.
 
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Some more...may I know how to identify the direction of tension BC vertically?
Thanks.
 

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frozen7 said:
Some more...may I know how to identify the direction of tension BC vertically?
I'm not sure I understand your question. The angle of BC is given. The tension pulls upwards at B and downwards at C.
 
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Thanks a lot...
 

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