Timeline for How can a massless string move if the net force on it is always zero?
Current License: CC BY-SA 4.0
12 events
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| Sep 20, 2020 at 16:15 | comment | added | Chris♦ | @illustro That is a conclusion in special relativity, not in classical mechanics. In any case, it doesn't apply to bound particles. | |
| Sep 20, 2020 at 15:57 | comment | added | illustro | The logical consquence of a massless object, it that it must be travelling at the speed of light. It has an accelleration of 0. van.physics.illinois.edu/qa/listing.php?id=1354 | |
| Sep 20, 2020 at 3:29 | comment | added | Chris♦ | @FilipMilovanović If you prefer you can take the limiting case as $m\to 0$. For any fixed acceleration, it is clear that $F\to 0$. So, again, regardless of acceleration, $F=0$ for a massless object. Regardless of acceleration, it is clear from Newton's second law that there can be no force. And it's easily justifiable as a limiting case of massive objects as $m\to 0$. | |
| Sep 20, 2020 at 1:13 | comment | added | Filip Milovanović | P.S. For clarity -by "practice" I mean "making predictions that can be experimentally tested"; I ran out of space in my previous comment. | |
| Sep 20, 2020 at 1:08 | comment | added | Filip Milovanović | This answer is somewhat misleading, IMO. I mean, saying that it's true for any value of $a$ is not really that much of a feature. You could say that the usefulness of it breaks down for zero mass, since you can't flip it around, and express it as $m \vec{a}/m = \vec{F}/m =\{\vec{a} \text{ for } m \neq 0 \text{, else undefined}\}$. You could legitimately decide to treat it as not applicable in this case. The reason an imaginary massless string can move is because it's a part of an idealized model that assumes that it can, without justification, and it works, both with the math and in practice. | |
| Sep 19, 2020 at 19:11 | comment | added | Post169 | @Drjh Let's say you have an imaginary basketball that is imaginarily glued to the palm of your right hand. To the extent that it exists, it will always move with your hand, yet it does not change the amount of force that your muscles exert at all. Thus, you can move it without exerting a nonzero net force on it. No physical object exists that is actually massless and can be regarded as being moved around; photons are massless, but the possibility of controlling the instantaneous location of one is limited. It just so happens that an imaginary object is consistent with the laws of physics here | |
| Sep 19, 2020 at 10:47 | comment | added | joseph h | @Chris your last comment confused me - how does an “object” with zero net force applied to it, which is massless accelerate? Can you give me an example please? | |
| Sep 19, 2020 at 10:19 | vote | accept | Jacob Gonzalez | ||
| Sep 19, 2020 at 10:19 | comment | added | Jacob Gonzalez | I see! That clears up my confusion then. Thank you for the answer! | |
| Sep 19, 2020 at 10:17 | comment | added | Chris♦ | A net force of 0 can cause an acceleration so long as the object is massless. | |
| Sep 19, 2020 at 10:06 | comment | added | Jacob Gonzalez | But isn't a net force needed to cause an acceleration? Or is it possible that a net force of 0 can still cause an acceleration? | |
| Sep 19, 2020 at 9:49 | history | answered | Chris♦ | CC BY-SA 4.0 |