The Most Mind-Blowing Aspect of Circular Motion

Comments · 4.2K

• QT

  @qtube2007 · 3 years ago

  and this is why all those Applied Math questions always stated " a non-elastic string" where they would assume the reaction to be instantaneous.

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• TI

  @Tim3.14 · 3 years ago

  The moral for physics teachers is “don’t forget to specify a massless string”😁

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• JH

  @JHBG1971 · 2 years ago

  A good summary would be to say that releasing the string <> releasing the ball. The ball isn't released until the tension wave reaches it and therefore continues its circular motion.

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• SC

  @SC-rb2jr · 2 years ago

  In a slingshot the ball is released from the ‘string’, so the expected path, a tangent to the circle is correct.

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• ER

  @Error6503 · 3 years ago

  This is why is in all physics exams I took the questions started with "Assume you have a system with friction-less couplings in a vacuum and a perfectly uniform spherical object connected by a rigid rod to an infinitesimally small single point" because once you have to take account of material tension, air resistance and even object widths then the question gets increasing more problematic to answer correctly.

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• PA

  @pataplan · 3 years ago

  In a way, this is a trick question. I think most people just think of the string being released from the center as being essentially the same problem as the ball detaching from the end of the string. If the problem were the latter, the ball detaching from the string, the answer would indeed be "b." The reason it's "a" is because no string is an infinitely rigid body, thus of course it would take a non-instantaneous amount of time (I imagine no faster than the speed of sound in the material the string is made of) for the ball to experience a change in centripetal force coming from the other end of the string. A question arises, what's the maximum angle that the ball can continue to subtend after the string is released? I'm guessing it's equal to the length of the string in the ideal case (that is to say 1 radian) but have no idea what it would be with the best real world material.

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• DH

  @dhavalmysore · 3 years ago

  Good presentation and the slow motion video with the slinky spinning the ball to illustrate the effect of finite time required to propagate the information was very impressive. I was thinking of the &quot;what if the sun disappears&quot; case as an example but you mentioned it at the end. That being said, the question in the beginning was tricky in the sense that most people would assume you are using &quot;fully rigid&quot; spring. Could have started off by clearly stating that the string is elastic, or by not even posing this question but just saying that this video demonstrates the effect of elasticity or finite speed of propagation of information on circular motion. That alone in itself is incredible in itself, as you have demonstrated in the rest of the video. <br>This tricky question at the beginning made it hard to take you seriously in the beginning, especially when it was immediately followed by the example of an object on a rotating turntable wherein the cause of the centripetal force is frictional force and the behavior you showed (it slipping and it following a curved path) was for a phenomenon not directly related to this topic, which you didn&apos;t even get into in the video! Could have avoided the sensationalism, just my opinion.

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• EB

  @ebenenspinne4713 · 2 years ago

  Weirdly, I think for many people who have some knowledge in physics, the earth-sun example is actually the most intuitive.

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• LA

  @lake5044 · 3 years ago

  Since we&apos;re not ignoring small details, the ball also has to rotate. We can explain it either as to conserve angular momentum since it will no longer be rotating, or because points in the ball have different speeds since they are at different distances from the center of the rotation.

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• ST

  @stevelc777 · 3 years ago

  The words &quot;immediately on release of the string&quot; do not mean the same as &quot;when the ball is let go&quot;. The ball is still under the force of the tension of the string all the while until the propagated wave reaches the ball. It is at this point that the ball is &quot;let go&quot; and then it turns out that B was indeed the correct answer, at this point in time.

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• HA

  @haroldwestrich3312 · 3 years ago

  Interesting perspective - I once did an experiment similar to this but I used a 9&quot; nail with feathers on it like an arrow and spun it at high speed by hand at about ten feet of line. I had set up a knife so that when I wanted to &quot;Release&quot; the nail, I would drop down a little at the knees and let the string be cut by the knife near the nail - worked great and it really was traveling at high velocity and I believe that the tiny speck of fishing line left beyond the knife was so minuscule that it had very nearly ZERO effect on the &quot;STRAIGHT&quot; trajectory of the nail - Your ball that continues on the circular path is interesting looking but, in effect, is simply NOT yet truly released from the force holding in in the circular pattern. The sliding puck was a similar case because - although the puck lost enough friction to slide; it was still, partially, being restricted by friction.

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• OB

  @obiwanduglobi6359 · 3 years ago

  Got me with that one. When the connection between the ball and the string is released, the correct answer is (b)... But the experiments were great anyway!

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