Today I was thinking a bit myself about circular motion and I come up with a doubt. Suppose we tie a string with a bob and then fix other end of the string with a support. Suppose the string is inextensible and if we give the bob a velocity then it will undergo a circular motion and the magnitude of its velocity will remain unchanged as the work done by the tension will be zero. But we can say the string is providing tension to the bob as the bob has the tendency to move forward straight which would have increased the length of the string, so to maintain its length it provides the tension in such a way that it's length remain fixed. The time of impact is very small because in the next moment it's velocity changes the direction so we can say that an impulse is given to the bob perpendicular to its motion to change its velocity but why it's magnitude doesn't change though the impulse is perpendicular

  • $\begingroup$ What impact? Please explain the situation a little more (maybe with a diagram). $\endgroup$ Commented Nov 24, 2018 at 18:28
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    $\begingroup$ Actually a diagram will be better but I dont know how to post a picture $\endgroup$ Commented Nov 24, 2018 at 18:36
  • $\begingroup$ If you add a link to an online image, and someone can edit your post to embed it inside. $\endgroup$ Commented Nov 24, 2018 at 20:22

1 Answer 1


Nice doubt. Really was once confused in this. The impulse provided by string continually changes direction. So it is going to be vector summed, but you considering its scalar sum. The long explanation So as the velocity is at a certain direction and tension is applied perpendicular giving it a simple perpendicular impulse so a slight component develops in a perpendicular direction and small about #Tdt#. So as very small variation in magnitude which we neglect but the velocity changes direction that's all.essentially the velocity changes direction and the next impulse (which we were calling the perpendicular impulse) will be perpendicular to the new velocity direction.This will make the velocity rotate by a small angle again. So you see the the net impulse is vector sum of all (T*d t) at different time intervals which continually changes direction and over one complete direction is 0. I hope you understand

  • $\begingroup$ So you got it?? $\endgroup$ Commented Nov 24, 2018 at 13:28
  • $\begingroup$ Yes it's ok that if we neglect the change in magnitude then there is no doubt in the further motion but let us in case assume that the change in magnitude in one impulse i very small but the bob is getting that kind of impulse so many times. Won't there be any finite change of magnitude after sufficient number of revolutions? $\endgroup$ Commented Nov 24, 2018 at 13:32
  • $\begingroup$ Sry i knew I messed up my answer so I edited it . Read it again $\endgroup$ Commented Nov 24, 2018 at 13:34
  • $\begingroup$ The impulses wil be changing direction not in a fixed direction but rather different direction $\endgroup$ Commented Nov 24, 2018 at 13:34
  • $\begingroup$ Which will be vector summer which will essentially be mathematics so I will have to show u calculations $\endgroup$ Commented Nov 24, 2018 at 13:35

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