Thursday

September 3, 2015
Posted by **Casey** on Saturday, June 28, 2008 at 3:31pm.

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**Casey**, Saturday, June 28, 2008 at 3:44pmDo you just find the resultant?

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**Casey**, Saturday, June 28, 2008 at 3:46pmNot sure how to go about adding up the angles. Thanks.

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**drwls**, Saturday, June 28, 2008 at 4:28pmYes, you find the resultant.

You don't add the angles. You add the components.

Letting north be the +y axis and east be the +x axis, the components of 7 m/s [N30E] are 7 sin 30 = 3.5 along the +x axis and 7 cos 30 = 6.062 along the +y axis. Calculate the components of the other vector similarly, and add the x and y components separately for the resultant.

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**Casey**, Saturday, June 28, 2008 at 6:42pmI need to use trigonometry to do this though, not component law, is there any real difference?

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**drwls**, Saturday, June 28, 2008 at 6:57pmIf you want the magnitude of the resultant, use the law of cosines. Drawing a figure will help.

The law of sines can get you the sine of any angle of the triangle formed by the two velocity vectors and the resultant.

it's easier using components, but the answer will be the same.

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**Casey**, Saturday, June 28, 2008 at 7:33pmYes, ok I have done that, I drew a diagram, but the thing is, you have all these angles, the co interior angles, and I am unsure of which ones to use - you have the 30 from the NE and then there is 17 from SE but there is another one, and I am not sure how to find it. I know the equation would be R^2= (7)^2 (2)^2-2(7)(2)COS___ I am unsure about what the angle is. Thanks, sorry for being so confusing.

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**drwls**, Sunday, June 29, 2008 at 3:18amI could write down a bunch of equations but since I cannot draw the triangle for you with the tools we have, it would probably be difficult to explain in words what is going on.

Draw the two velocity vectors end to end. Then close the triangle to get the resultant. The angle between the two vectors that you were provided is 47 degrees. That is the angle that you use in the law of cosines to get the magnitude of the resultant

R^2 = 7^2 + 2^2 - 2*14 cos 47 = 33.90

R = 5.82 m/s

Now use the law of sines to get the other two angles of the triangle, which will tell you the direction of the resultant.

It is a lot easier adding components.

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**Derek**, Sunday, June 29, 2008 at 10:55amThanks! I agree about the components but the question said specifically to use cosine and sine law :(. Thanks very much though!

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**Derek**, Sunday, June 29, 2008 at 11:03amI used the sine law to get 15 degrees. I am unsure as to what I now do with this angle. Do I add or subtract it somewhere to find the overall angle I am looking for? Thanks again! I owe you!