I am very new to physics. This evening, I was reading about the concept of objects to be considered as point objects under some circumstances. And I was trying to think of circumstances under which the extended objects no more needed to be considered point objects. I believe I couldn't go far. Would you guys help!?
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$\begingroup$ I think you are asking us to teach you physics, and to have a discussion with you. That kind of help should be available from your teacher and/or classmates. Failing that, there are many other resources available over the internet. You are expected to make use of such resources before asking Physics Stack Exchange for help. $\endgroup$– sammy gerbilCommented Jun 21, 2016 at 15:47
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2 Answers
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An extended object can be considered a point object or simply a particle whenever its dimensions are negligible compared to the characteristic dimensions involved in the problem and it does not have an internal structure (or it may be also neglected). For example, in studying the motion of the Earth in the solar system we can treat the planet as a point mass. Its radius is totally neglected compared to characteristic lengths such as the distance to the Sun. The Earth's fluid motions are also negligible and will not interfere appreciably in its revolution around the Sun.
answered Jun 20, 2016 at 16:29
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$\begingroup$ Does the last sentence of urs imply that earth's fluid motion actually can interfere in the motions of nearby planets? $\endgroup$ Commented Jun 20, 2016 at 17:36
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2$\begingroup$ @user108579 Yes, it can. However effect is small compared to rest of the earth, so we ignore it and avoid calculations. In most cases, answers like 6 work out, instead of something like 6.00001 etc. So, we avoid calculating them. But some people find it necessary in their jobs to use better models than a point like earth, for which they're usually paid. $\endgroup$ Commented Jun 20, 2016 at 17:44
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You can consider an object to be a point particle, when you only think of its translatory motion, means it is moving in 3-D like a point would, like, if a point particle moves around, it is so small that you don't care about its orientation, whether it is rotating. So, when you are studying objects like a block sliding down an inclined plane or something, you know its orientation won't change, it won't expand or rotate etc., so if you replace it by a point of the same mass,and in that situation, if you find out the motion of the point particle it will move the same way to a great extent like the block would.
Physicists use such approximations all the time to save time and to avoid a lot of mathematics by using simpler models, such that you get a pretty accurate answer out of the situation by avoiding unnecessary calculations that will contribute very less to the final observation number.
Get used to this, you'll be doing this in your whole syllabus, while treating electron as a point charge, a thermodynamic process as adiabatic etc. The reasons for choosing a particular mathematical model will be different in all scenarios, like sometimes what you're studying is too small, and sometimes you know that constraints are making your object like a point particle, but in both cases the same model would work and would give you pretty good approximations. Calculations are HARD and boring(sometimes).
answered Jun 20, 2016 at 16:38