We all have done this!

The situation in the question has been experimented my most of us in our childhood, but no one probably thought of this!

There is a thread of mass M M and length L L is stretched on a smooth horizontal surface with constant forces acting on either sides of the rope. A sudden movement causes the rope to form a circular loop traveling traveling from one end of the rope, with constant velocity v v .

If:

  • v v is the velocity of the loop.
  • \ell is the angular momentum of the moving loop about the centre of the loop.
  • K E KE is the Kinetic Energy of the loop.

Evaluate v + + K E \large |v| + |\ell| + |KE| . Give your answer to 3 decimal places.

Details

  • Take M = 5 k g , L = 3 m , F = 7 N M = 5kg,~ L = 3m,~F = 7N
  • Radius of the loop, R = 10 c m R = 10 cm
  • There is no gravitational field.

Source: 200 Puzzling Physics Problems


The answer is 6.6622315032045300708397782678424.

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2 solutions

As no one has given a solution till now, I am giving now.

If we consider the motion of the loop in the centre of the loop frame, the loop performs uniform rotation. With this in mind, we can use

T = ρ v 2 T = \rho v^2

where T T is the tension, ρ \rho is the linear mass density, and v v is the speed of all elements of the loop.

Now, we know F = T F = T

v = F L M \Large \Rightarrow \boxed{v=\sqrt{\dfrac{FL}{M}}}

So that = I ω = 2 π R ρ R 2 v R = 2 π R 2 M F L \begin{aligned}\ell &= I\omega\\&=2\pi R\rho R^2 \dfrac{v}{R}\\&=2\pi R^2\sqrt{\dfrac{MF}{L}}\end{aligned} = 2 π R 2 M F L \Large\Rightarrow \boxed{\ell = 2\pi R^2\sqrt{\dfrac{MF}{L}}}

And, K E t o t = I ω 2 = 2 π R ρ v 2 = 2 π ρ R F ρ \begin{aligned}KE_{tot} &= I\omega^2\\&=2\pi R \rho v^2\\&=2\pi\rho R \dfrac{F}{\rho}\end{aligned} K E = 2 π R F \Large \Rightarrow \boxed{KE=2\pi RF}

v + + K E = F L M + 2 π R 2 M F L + 2 π R F \large\color{#3D99F6}{\therefore\boxed{v+\ell+KE = \sqrt{\dfrac{FL}{M}}+ 2\pi R^2\sqrt{\dfrac{MF}{L}}+2\pi RF}}

Moderator note:

This is the best kind of problem, a simple situation everyone has seen but few have calculated. How would this change in the presence of a gravitational field?

But I hope you give the final value too

Nikhil Poojary - 5 years, 8 months ago

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Why? You can find it by substituting right?

Kishore S. Shenoy - 5 years, 8 months ago

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Was a very easy question, got it in 1st attempt. but as I say , every thought is unique and classy. :) . Thumbs up. keep posting.

Pranjal Prashant - 5 years, 8 months ago

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@Pranjal Prashant Thanks! Sure!

Kishore S. Shenoy - 5 years, 8 months ago

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@Kishore S. Shenoy Can you post a level 5 questions for ninth graders (well, I have joined fiitjee classes so can you just make it difficult but please for a ninth grader)

Department 8 - 5 years, 8 months ago

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@Department 8 well I have seen few questions that are really easy. For instance question titled abstract pac man . My questions, first mechanics problem, awa others are very easy to solve too. Though they have level 4 rating as many people solved it.

Pranjal Prashant - 5 years, 8 months ago

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@Pranjal Prashant well thanks

Department 8 - 5 years, 8 months ago

@Kishore S. Shenoy 400 points now

Pranjal Prashant - 5 years, 8 months ago

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@Pranjal Prashant ?? Why did you mention that now?? I think that's the maximum a question can get in Brilliant

Kishore S. Shenoy - 5 years, 8 months ago

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@Kishore S. Shenoy yes, it is max. well this one was deserving it. I can easily solve the difficult problems (almost :p) . But posting such good ones is a nightmare. Tried to post my own difficult maths problem but it had so many typing and formatting errors that I had to delete it.

Pranjal Prashant - 5 years, 8 months ago

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@Pranjal Prashant Hehe, happened to me once or twice. One is just happening... Check this one

Kishore S. Shenoy - 5 years, 8 months ago

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@Kishore S. Shenoy Sorry for the late reply. There were exams.... . And I solved that question too. Meanwhile I was going through your physics set and it's one of the best after rounak's level 5 and aman's hot integral. P.s. I have completed hot integrals. First one to complete the set . Feeling blessed. :)

Pranjal Prashant - 5 years, 7 months ago

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@Pranjal Prashant You haven't solved mine though 😛😜

Kishore S. Shenoy - 5 years, 7 months ago

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@Kishore S. Shenoy I will solve them when I login on pc. There is some problem in posting answer on my mobile device.

Pranjal Prashant - 5 years, 7 months ago

@Kishore S. Shenoy And just looking at the tabs, I want to know how was your exam ? I am getting 69. Made lots of silly mistakes there.

Pranjal Prashant - 5 years, 7 months ago

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@Pranjal Prashant You got good. I wrote Online Exam, so... you know right?

Kishore S. Shenoy - 5 years, 7 months ago

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@Kishore S. Shenoy Know what? ??

Pranjal Prashant - 5 years, 7 months ago

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@Pranjal Prashant I don't know which were the answers I marked. So, I can't get marks... ¨ \ddot\frown

Kishore S. Shenoy - 5 years, 7 months ago

@Kishore S. Shenoy Solved every question except the day dreamer. It is deleted ¿¿ :)

Pranjal Prashant - 5 years, 7 months ago

@Pranjal Prashant Try my New Earth question!

Kishore S. Shenoy - 5 years, 8 months ago

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@Kishore S. Shenoy Done ! And one more thing, you are waiting for which moment? Level 5 in number theory?

Pranjal Prashant - 5 years, 8 months ago

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@Pranjal Prashant And Level 5 in Electricity and Magnetism. Studying for it! I don't know anything on it! BTW, are you a moderator here?

Kishore S. Shenoy - 5 years, 8 months ago

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@Kishore S. Shenoy NONONONO, I am not a moderator. Just a good solver. Posting up questions or editing them is miles out of my way. And perhaps you are in 11. Otherwise you easily would have level 5 in E&M.

Pranjal Prashant - 5 years, 8 months ago

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@Pranjal Prashant Yup. I'm in 11th and in 12th you learn E&M! Nice to meet you! You are in 12th?

Kishore S. Shenoy - 5 years, 8 months ago

@Pranjal Prashant Why don't you try solving A long Slipway?

Kishore S. Shenoy - 5 years, 8 months ago

@Calvin Lin , sir, if you are the Challenge Master, when there is gravitation, the net force will not be zero and thus we can't use the first equation.

Kishore S. Shenoy - 5 years, 8 months ago

Source of the question ?? And please give computable figures, calculation was humongous.

vidhan singh - 5 years, 4 months ago

how did u write the first equation

rakshith lokesh - 3 years, 3 months ago

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Really sorry for replying late. I was busy with my courses. Check out the solution for this problem for the derivation of that equation.

Kishore S. Shenoy - 2 years, 10 months ago
Shubham Maurya
Nov 29, 2015

Does this really happens?? I have never seen it in reality!

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