- #71
TSny
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So, since you got the angles correct, your diagram hopefully looks something like this:
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TSny said:[EDIT: Sorry, Vibhor, your values for the angles are correct! When I worked through this part of the problem, I did not use the unit vectors ##\hat{r}## and ##\hat{\phi}##. When I read your last thread, I had the vectors pictured in my head in their opposite directions.]
Great! The first two equations above can pretty much be read directly off the diagram. But using the unit vectors is good, too.Vibhor said:##-dscos\phi = dr ##
##-dssin\phi = rd\phi ##
From this I get ##\frac{dr}{r} = \frac{d\phi}{tan\phi}##
TSny said:Perhaps the curve will be more recognizable when expressed in terms of the Cartesian coordinates of the particle (x, y).
TSny said:It doesn't make any difference as long as you make your definitions clear.
TSny said:Does the particle end up where you expect it would?
TSny said:Can you relate C to the place where the particle crossed the x-axis in going from quadrant II to quadrant III?
TSny said:In thefirstsecond quadrant an infinitesimal amount of tension will set the particle in motion. If the tension is kept essentially at zero as the particle moves at a slow constant speed in thefirstsecond quadrant, what direction does the kinetic friction need to act so that the net force is zero? If you know the direction of the kinetic friction, what can you say about the direction of the motion?
I had to think about this, too. I attempted an explanation in post #42. I'll let you think about it some more. I'm shutting down for the night. Good work!Vibhor said:Back to second quadrant .
I am still bemused by the fact that even if the string is pulled at some angle to the +y direction ( even though it is a very small magnitude Tension force ) at t=0 when the particle was at rest in second quadrant , the particle instead of going radially inwards towards the hole starts moving down slope .
Please explain it a bit more .
With a bit of geometry, you can get the curve fairly easily from TSny's excellent diagram in post #71.Vibhor said:Ok . Good point .
I don't know . Even though I have got the equation of the curve , I can't imagine how the particle is actually moving (except it is moving in a circular fashion not centered at the hole) . I am still not sure how to determine constant ##C## .
Nice.haruspex said:With a bit of geometry, you can get the curve fairly easily ...
Wow . Enjoy and be safe.(cycling Berlin to Copenhagen)
TSny said:I had to think about this, too. I attempted an explanation in post #42. I'll let you think about it some more.
I assume you are referring to the case where the mass starts at a level above the hole. For any actual nonzero tension in the string, the motion of the mass will have a horizontal component towards the hole. But in the limit, as the tension tends to zero, that component tends to zero.Vibhor said:b) As soon as tension exists in the string , the equilibrium of particle is disturbed which causes it to move down slope . But why down slope ?
TSny said:In thefirstsecond quadrant an infinitesimal amount of tension will set the particle in motion.
That can be answered by going back to the equation that allowed for arbitrary tension and acceleration you had before TSny joined the thread and generalising it to the case where the string is at some angle to the horizontal. You can take the limit as acceleration tends to zero and see that (where the particle is further up the plane than the hole) the path tends to the 'vertical'.Vibhor said:And why not vertical component also tend to zero and particle stays at rest instead of moving down slope ?
Suppose the particle does not move parallel to the y-axis when in the second quadrant. What direction would the friction force point? Could the forces sum to zero in this case?Vibhor said:Could you please explain how does tension (acting at an angle) causes the particle to move down slope . I have thought about it but I am still unsure .