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Jan 14, 2008

2/203

I am having some trouble getting started on this problem, especially deciding on what angles to use for the law of sines/cosines. Any help would be much appreciated
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4 comments:

JFR said...

I would start by writing down the absolute velocities of the vehicles in i/j coordinates (can be computed without the Law of Sines or Cosines - you just need to use the given angle). From there you can compute the relative velocity, which the problem asks for, and equate the vector result to the velocity equation associated with polar coordinates to complete the problem. Hope that helps.

January 15, 2008 at 9:51 AM
Lindsay said...

I'm not quite sure how to find r_dot and theta_dot. I end up with one equation(the velocity one) and 2 unknowns. Any suggestions?

January 15, 2008 at 6:45 PM
Kard16 said...

you actually have 2 equations and 2 unknowns. the velocity equation should have an i and j component. If you set that equation equal the the velocity equation for polar coordinates and substitute e_r and e_theta with it's i and j equivalent (use the angle to solve for these), you can set your i's and j's equal and you'll get 2 equations where r_dot and theta_dot are unknown.

January 15, 2008 at 8:01 PM
John said...

I believe that this problem is looking for terms in polar coordinates. Therefore, the easiest way to achieve workable numbers is to convert the only cartesian value that you are given (the velocity of B, 60 km/hr in the i direction) and convert that to polar coordinates. This can be done by doing the dot product of v_B with e_theta and with e_r.

e_r = -sin(theta)i + cos(theta)j
e_theta = cos(theta)i + sin(theta)j

January 15, 2008 at 9:00 PM

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