Timeline for If Newton's third law is true, why can we sink in sand?
Current License: CC BY-SA 4.0
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| May 3, 2020 at 5:56 | history | edited | knzhou | CC BY-SA 4.0 |
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| Nov 17, 2018 at 13:48 | comment | added | ffahim | @knzhou then explain why a book on a table is at rest.? | |
| Jun 3, 2018 at 3:38 | comment | added | candied_orange | @zwol Archimedes reportedly said, "Give me a lever and a place to stand and I will move the earth". What he didn't realize is that he already had both. His legs were his lever. He moved the earth every time he got up out of bed. Just not by much. | |
| May 30, 2018 at 18:13 | comment | added | tonysdg | Relevant XKCD: xkcd.com/162 | |
| May 30, 2018 at 16:11 | comment | added | JimmyJames | @PeterA.Schneider You're right, I should not have said 'net force' at least with respect to the marble. There is a net force on the sand, however. My point is that your comment suggested that all you would need is some initial force to cause the sinking but that is not the case. Without a continually applied force, things will not sink in sand. | |
| May 30, 2018 at 15:30 | comment | added | Peter - Reinstate Monica | @zwol The earth thing is an important point. It is ridiculous that wikipedia featured Newton's cradle as an example for the preservation of momentum before I it was removed after my critique. (I admit I should have removed it myself but was not one of the pages previous editors.) | |
| May 30, 2018 at 15:26 | comment | added | Peter - Reinstate Monica | @JimmyJames Yes, it requires a force; no, it does not require a net force. The same force I'm exerting on it is exerted by friction on the marble in the opposite direction; that is why it doesn't accelerate, once it's started moving. It's a similar situation a person experiences falling in the atmosphere at terminal velocity. S/he is in force equilibrium between friction and gravity. | |
| May 30, 2018 at 15:08 | comment | added | JimmyJames | @PeterA.Schneider Try pushing a marble horizontally in sand. You will find it requires a constant net force. | |
| May 30, 2018 at 14:21 | comment | added | zwol | Also, what Newton's Third Law does mean is that when something starts moving, something else must have started moving in the opposite direction. The trouble is that in the environment familiar to us, very often the something else is the Earth, whose inertia is so immense that its velocity does not appear to change. | |
| May 30, 2018 at 13:50 | history | edited | knzhou | CC BY-SA 4.0 |
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| May 30, 2018 at 13:17 | history | edited | knzhou | CC BY-SA 4.0 |
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| May 30, 2018 at 13:17 | comment | added | Peter - Reinstate Monica | Yes ;-)... Yes, seriously. That distinction is important because it is not intuitively clear that there is zero net force when sinking into sand. | |
| May 30, 2018 at 13:06 | comment | added | Peter - Reinstate Monica | Note that "falling" with constant speed -- well, it's not physically free falling, so let's call it sinking, like sinking into sand or a fluid -- somewhat counterintuitively does not require a force difference, since the motion does not change. Newton's first law itself dictates it: "perseverare in statu suo quiescendi vel movendi..." (bodies "stay in their state resting or moving...") unless an outside (net) force acts upon them. It's just the acceleration in the beginning which needs a force. | |
| May 30, 2018 at 11:36 | history | answered | knzhou | CC BY-SA 4.0 |