Feb 24, 2008
3/160
I've been working on this problem and I understand the basic kinetics of it ok, but when it comes to the kinematics of it I get a little thrown off. I know I need to use a relative velocity equation to find the velocity of v_b and i think it should be v_b=v_a + v_b/a but how do i find v_b and v_b/a in this instance?
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9 comments:
You really don't need any of that. What is important is for you to know what is happening at both instants t1 and t2. It would be best to follow Prof. Krousgrill's advice to select the whole link as your system, that way it is much easier.
You should see that only gravity is acting on the system, thus there is no U(nc) term. At t1 A and B have no kinetic energy but have gravitational potential energy. At time t2 if you notice well you'll see that all the potential energy has become kinetic energy. One thing to note is that at t2 vb is ZERO, can you see why?
One more thing, can someone confirm v_a = 5.29 m/s? ThAnKs!
Va is not 5.29 m/s . It should be 2.3 m/s. If you can write out your equations in here I can try and help you out.
Emily you're right the answer is 2.3 m/s. I got 5.29 m/s because I forgot to take the square root :S. Thanks a ton for the reply and for offering to help ;).
You're welcome! Glad it worked out!
Nevermind, copied some dimensions wrong on my figure.
I wasn't sure what to use for the height values. Do we just measure our h from where we drew the datum?
Yes, that is correct. Be sure to correctly account for the sign: if below the datum line, V is negative at that position, and if above the datum line, V is positive at that position.
My suggestion on this one is to put the datum line through O and use this line for both A and B (simpler trig that way).
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