• Feb 8th 2011, 11:28 AM
Barthayn
Got a lesson with vector addition and I need help with the last question.

17. Two forces with magnitudes 8 N and 11 N act on a large object. The angle between the forces is 30^o.
a) Draw a diagram to represent the combine effect of the forces.
b) Calculate the magnitude of the resultant force.

I have no idea on to do both. I drew a free body diagram and then used c^2 = a^2 + b^2. But that was correct. Then I used my law of cosines and that didn't work. Then I used my law of sines and that didn't work.

Could someone help me with a diagram. I think that will be best for me because I cannot interpret the question correctly.
• Feb 8th 2011, 11:33 AM
topsquark
Have you tried the component method to add the vectors?

-Dan
• Feb 8th 2011, 11:35 AM
Barthayn
We were not taught that yet. We're only 2 days in the course learning new stuff. The only thing that we know is that we have to add them together. However, I get the wrong answer of 5.7 N while the book claims it is 18.4 N.
• Feb 8th 2011, 12:15 PM
topsquark
Vecotrs
Try the Law of Cosines again.

-Dan
• Feb 8th 2011, 12:20 PM
Barthayn
Quote:

Originally Posted by topsquark
Try the Law of Cosines again.

-Dan

Thank you. I tried that, however, forgot that two negatives make a positive. Lulz! Thanks. However, how why is it 150^o than 30^o? Why did we have to take 180 and substract the given angle? Wouldn't it the 30 degress be where the 150 degrees is?
• Feb 8th 2011, 03:27 PM
skeeter
Quote:

Originally Posted by Barthayn
However, how why is it 150^o than 30^o? Why did we have to take 180 and substract the given angle? Wouldn't it the 30 degress be where the 150 degrees is?

The 30 degree angle is betwen the two vectors when they are arranged tail to tail. Vectors are added head to tail ... the 8N vector was translated up to the position shown in topsquark's excellent diagram. The interior angle of the resulting triangle is 150 degrees.

http://www.physchem.co.za/Vectors/Graphics/GRD20012.gif
• Feb 8th 2011, 09:47 PM
JesseLMackey