Timeline for Why doesn't a braking car move backwards?
Current License: CC BY-SA 4.0
24 events
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| Jun 25, 2023 at 13:16 | comment | added | tparker | I think it's worth clarifying that frictional forces reduce relative motion between two surfaces in contact. If you slowly pull a tablecloth out from a loaded table, then the friction between the cloth and the plates will cause the plates to speed up (in your frame of reference), not slow down. The fact that the direction of frictional forces depends on relative rather than absolute motion follows from the Galilean invariance of Newtonian physics, but it may not be obvious to a beginner. | |
| Dec 10, 2018 at 14:48 | comment | added | rebusB | @user71659 - if your car was moving 0.0000000000000001 mph would you notice or would you consider it stopped? | |
| Dec 10, 2018 at 11:53 | comment | added | Peter - Reinstate Monica | @Draco18s There are different types of friction; dynamic dry friction (e.g. skidding wheel on street) does not depend on the speed, so it is purely additive (but vanishes when the car comes to a stop, which is why the car doesn't start going backwards). | |
| Dec 9, 2018 at 22:55 | comment | added | vol7ron | “But they also slow down motion that produces reverse movement” I feel this comment is confusing (irrelevant) to the question. There is no negative force under review. Friction doesn’t cause negativity, it only reduces positivity; like hitting a puck on ice vs asphalt. | |
| Dec 9, 2018 at 6:36 | comment | added | corsiKa | @DanStaley Zeno would agree with you. | |
| Dec 9, 2018 at 4:32 | comment | added | zahbaz | @Krumia Funny, I pictured the spring attached to the rear of the car. | |
| Dec 8, 2018 at 0:20 | comment | added | user71659 | @DanStaley You can't stop a car unless it's at absolute zero... | |
| Dec 7, 2018 at 20:45 | comment | added | JiK | @Draco18s How does introducing "multiplicative" simplfy anything? | |
| Dec 7, 2018 at 18:58 | comment | added | Draco18s no longer trusts SE | @JiK What part of "simplified analogy" wasn't clearly communicated? | |
| Dec 7, 2018 at 18:46 | comment | added | JiK | @Draco18s In what way is friction multiplicative? | |
| Dec 7, 2018 at 5:37 | vote | accept | Harnoor Lal | ||
| Dec 6, 2018 at 20:26 | comment | added | Draco18s no longer trusts SE | @DanStaley Oh its definitely a simplification. :) (and you can multiply by 0.9 enough times to get to effectively 0, as the value is so small as to be meaningless over the timescales involved, but point taken). | |
| Dec 6, 2018 at 19:54 | comment | added | Dan Staley | @Draco18s An interesting simplification, although if true it would mean you can't stop your car with the brakes (no amount of multiplying by 0.9 will ever make a positive number zero either)! | |
| S Dec 5, 2018 at 22:52 | history | suggested | CJ Dennis | CC BY-SA 4.0 |
Remove conditional
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| Dec 5, 2018 at 22:24 | review | Suggested edits | |||
| S Dec 5, 2018 at 22:52 | |||||
| Dec 5, 2018 at 21:56 | comment | added | Draco18s no longer trusts SE | Essentially: braking (friction) is a multiplicative. No amount of multiplying by 0.9 will ever make a positive number negative. Rockets, on the other hand, are additive (and you can absolutely add a negative number to a positive one and end up with a negative number). | |
| Dec 5, 2018 at 15:44 | history | edited | Chemomechanics | CC BY-SA 4.0 |
deleted 1 character in body
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| Dec 5, 2018 at 15:43 | comment | added | Bradley Uffner | @OscarBravo SpaceX's high speed u-turn that the first stage booster's do when going back to the landing pad, is a great example of exactly what you are talking about. | |
| Dec 5, 2018 at 15:25 | comment | added | Oscar Bravo | This is how a spacecraft slows down. It rotates so that its rocket nozzle is pointing in the direction of motion. Then it fires the rocket to produce a force and decelerate. If it kept the motor burning, it would eventually stop and then accelerate back the way it came. | |
| Dec 5, 2018 at 12:54 | comment | added | infinitezero | Or just use a turbine that accelerates you backwards as breaking mechanism. | |
| Dec 5, 2018 at 7:12 | comment | added | sampathsris | Thanks for the mental image of a car slamming into a giant spring. | |
| Dec 5, 2018 at 5:37 | comment | added | Nelson | A better example would be crashing into a very solid barrier. The car will bounce backwards... | |
| Dec 5, 2018 at 2:24 | comment | added | Mazura | Acceleration is "the rate of change of velocity per unit of time" . It is not a force-applying [mechanism]. +1 | |
| Dec 5, 2018 at 0:44 | history | answered | Chemomechanics | CC BY-SA 4.0 |