Solving a Tension Angle Physics Problem with Three Equations | Physics Forum

[frozen7]

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 can't get the answer: ABsin60 + BCsin30 = 2g
CDsin70 + BCsin30 = m(Q)g
ABcos60 = CDcos70 + BCcos30
Can anyone help me to solve this question? Thanks.

 

[Doc Al]

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.

 

[frozen7]

The part I don't understand the most is about the horizontal force...
Is ABcos60 = BC cos30 and BCcos30 = CDcos70 ??

 

[Doc Al]

Is ABcos60 = BC cos30 and BCcos30 = CDcos70 ??

That looks correct to me.

 

[frozen7]

Some more...may I know how to identify the direction of tension BC vertically?
Thanks.

 

[Doc Al]

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.

 

[frozen7]

Thanks a lot...