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In which of the following body can be considered point object

  1. A railway carriage moving without jerks between two stations.

  2. A monkey sitting on top of a man cycling in a circular path.

  3. A spinning cricket ball that turns sharply on hitting the ground.

  4. A tumbling beaker that has slipped of the edge of the table.

It has more than one correct answer. Please give reasons for your answer as well.

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  • $\begingroup$ Hi Arun, the etiquette of this site is that you shouldn't simply post homework questions or test questions- you should try to solve the problem yourself, and ask for help when and where you get stuck. What is your specific difficulty with the question? $\endgroup$
    – Professor Sushing
    Commented Oct 8, 2019 at 12:32
  • $\begingroup$ Hello Maco Ocram, actually I am a littlle confused with the concept of definite sized bodies being assumed as point object. One school of thought says a body can be assumed as a point object when it travels distance much greater than its size. Another says a definite sized body can be assumed to be a point object when it is in such motion that each of its particles are under same displacement, velocity and acceleration. In above question, nothing can be said about distance travelled. Though in option 1 and 2 each particle is under same acceleration and velocity. $\endgroup$
    – Arun Arora
    Commented Oct 8, 2019 at 13:00
  • $\begingroup$ This looks like an AP Physics 1 question. $\endgroup$
    – David White
    Commented Oct 8, 2019 at 15:40

1 Answer 1

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In 1) railway carriage is small compared to the distance between two stations so it can be considered as point object.

In 2), even though a monkey is comparable in size with a man, it is not comparable in size with a circular path, therefore it can be approximated as a point mass.

In 3) spinning cricket ball is comparable to the distance through which it turns sharply on hitting the ground, therefore it cannot be considered as point object.

In 4) beaker is comparable in size with the height of the table, so it too cannot be considered a point object.

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  • $\begingroup$ How do you know that In 2) size of monkey is very small comparatively, may be circular track is of radius = 1 m. $\endgroup$
    – Arun Arora
    Commented Oct 8, 2019 at 13:04
  • $\begingroup$ yes, it can be, but the whole question is nonsense if you look at it that way. somehow makes sense that circular path is much bigger in size than monkey. $\endgroup$
    – Luka Mandić
    Commented Oct 8, 2019 at 13:11

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