Wikipedia:Reference desk/Archives/Science/2019 October 7

= October 7 =

Thought Experiment / Equivalence Principle
Diagram on the right shows the elevator used in the thought experiment by the Einstien for equivalence principle

Elevator at rest on earth: Light beams or pulses emit at A and D at the same time, respectively. Both beams/pulses reach their destination B and C at the same time.

Elevator accelerating @ 9.8 m/s/s in free space: Light beams or pulses emit at A and D at the same time, respectively. Both don't reach their destination B and C at the same time. Pulse on right of observer reaches C faster as the floor of the elevator approaches it than pulse on the left where pulse moves away from the floor of the elevator when the floor tries to approach it (difficult to catch) - Right

So should the distance covered by pulses/ beams in time "t" be the same in both aforementioned cases?

'''Same scenario but with light clock '''

A light clock installed in the elevator of Einstein consists of two mirrors, M1 (attached to the ceiling) and M2 (attached to the floor) facing each other at a reasonable height "h" apart. Would the bouncing pattern of light beam/ pulse in between M1 and M2 be the same (equivalent) when the elevator is at rest on the ground and moving upward @ the rate of 9.8 m/s/s therein free space away from the local attraction of gravity?Eclectic Eccentric Kamikaze (talk) 02:20, 6 October 2019 (UTC)


 * Your assumptions are invalid to begin with. See relativity of simultaneity. This gets really complicated when you start involving acceleration. Someguy1221 (talk) 05:57, 6 October 2019 (UTC)
 * I dunno what your knowledge of physics is, but it seems you are considering way too hard problems for you. Considering your previous question above, it seem you miss, like, a couple of thousands hours of training in math and physics before you deal with a thought experiment that comes after Michelson–Morley experiment about speed of light. We can provide only so much help here. Gem fr (talk) 12:31, 6 October 2019 (UTC)
 * The article Einstein's thought experiments includes the perception by a man in an accelerating elevator that he is in a uniform gravitational field. Far from being mistaken, he only confirms the Equivalence principle that gravitational mass and inertial mass are the same. The Michelson–Morley experiment experiment replaces the imagined elevator with the actual accelerating (orbiting) Earth and measures velocities of light in opposite directions, consistently finding that paths A->B and D->C in the elevator have the same times regardless of any acceleration of the elevator. Einstein's deduction is that "rays of light are propagated curvilinearly in gravitational fields." DroneB (talk) 13:44, 6 October 2019 (UTC)

It just a simple question of equivalency whether it exists or not in all the above cases. I mean it’s a Yes or No question.

The feeling of a hanging person from the ceiling of the elevator would be different if the elevator is at rest on the ground and accelerating upward @ 9.8 m/s/s – Due to the direction of his weight. There are lots of other examples such as bouncing sequence (up and down) of the tennis ball on the floor of the elevator when it is at rest on the ground and accelerating upward.

Therefore I would say there is no such gravitational field exists inside (just a free space – ignoring Newtonian) the aforesaid elevator, however, one may feel gravity but upon touching elevator or any of its part in a direction other than the direction of acceleration.

So I am thinking why would gravity bend light when it does not even exist (indirectly equivalency) inside the free space of the aforementioned elevator as explained above? — Preceding unsigned comment added by Eclectic Eccentric Kamikaze (talk • contribs) 23:00, 6 October 2019 (UTC)
 * Do you even read answer already given to you?
 * That elevator accelerates at a constant rate forever no matter what happens, so the observer just sees two objects falling and hitting the floor like normal. already told you 00:08, 5 October 2019 (UTC)
 * There is absolutely no way for a man inside a box to know what the source of his acceleration is, ie, whether the acceleration is caused by gravity or some rocket booster or whatever just doesn't matter. Now if you are not trolling, you are good at faking it. Gem fr (talk) 01:14, 7 October 2019 (UTC)
 * I believe that all threads thus far begun by EEK are an extended exercise in argument from incredulity. Physics and algebra do not make sense to EEK, and so he concludes that there is something wrong with them. Until he accepts that he does not understand their most basic principles, it will not be possible to educate him. Someguy1221 (talk) 09:12, 7 October 2019 (UTC)

How many watts per kilo of building is each mercalli scale number?
Apples to apples and ratios instead of absolute if needed. Sagittarian Milky Way (talk) 15:57, 7 October 2019 (UTC)


 * I presume that we're talking the Modified Mercalli intensity scale. It is an observational scale, although it does have equivalent peak ground acceleration and peak ground velocity values associated with it in order to help construct USGS "shake maps" by combining observations with those from seismometers - see here. Mikenorton (talk) 16:06, 7 October 2019 (UTC)

If the butt of a rifle were so small as the bullet, how much damage would shooting it cause to the shooter?
If the butt of a rifle were so small as the bullet, how much damage would shooting it cause to the shooter? --C est moi anton (talk) 16:57, 7 October 2019 (UTC)
 * Not as much as you're probably thinking.
 * The recoil will depend on the impulse (physics), which is a measure of momentum transferred. There's also a conservation of energy. More is at our article on recoil.  But what you'll learn from that is that rifle recoil is survivable (in a way that bullet impact isn't) mostly because of the much greater mass of the rifle and stock, compared to the bullet, rather than it simply being spread over the area of a larger butt pad. Andy Dingley (talk) 17:14, 7 October 2019 (UTC)


 * (ec) There are two factors involved:


 * How compressed the force is per area, that is, pressure. The rifle should actually exert more force than the bullet, since some of the bullet's force is lost due to air resistance, and the rifle is also accelerated by the gases expelled with the bullet.


 * How compressed the force is in time, that is, impulse. Here, the rifle accelerates backwards more slowly, due to it's higher mass (F=ma), so the force it exerts on the shoulder is spread out over time more. This allows time for the body to respond. If you look at high-speed photography of a bullet shot at a rubber sheet, it fractures like glass, while at lower speeds it bends and stretches to absorb the blow. See impact (mechanics) for a description of this effect. SinisterLefty (talk) 17:20, 7 October 2019 (UTC)


 * Note the assumption that the rifle has far more mass than the bullet. In the case of a rocket propelled grenade, the opposite is true, and if not open in the back, the canister would fly backwards at a deadly speed. SinisterLefty (talk) 18:53, 7 October 2019 (UTC)
 * A real RPG is, as the name suggests, rocket-propelled. In other words, the tube is just for guiding it during launch. The speed comes from the gas ejected from the back of the rocket, not from the compressed gas in the tube. In a perfect world, there would be no action on the tube at all. Also, for most RPGs, its not the kinetic energy that makes them effective, but the explosion of the payload. --Stephan Schulz (talk) 19:11, 7 October 2019 (UTC)


 * It needs enough thrust to get it out of the tube, and at a velocity that won't allow the front end to drop. Based on the mass and speed required, that alone is a fair amount of thrust. SinisterLefty (talk) 20:26, 7 October 2019 (UTC)
 * Not to be confused with a recoilless rifle, which does not use rockets (and so might be a bit more relevant to this question). Alansplodge (talk) 20:35, 7 October 2019 (UTC)
 * Or better yet a mortar. I wouldn't suggest holding one in your lap as you fire it. SinisterLefty (talk) 20:43, 7 October 2019 (UTC)
 * But a mortar recoils, while rocket propelled grenades and recoilless rifles don't, at least on the firer. Alansplodge (talk) 21:32, 7 October 2019 (UTC)


 * Yes, and the OP was on recoil. SinisterLefty (talk) 22:28, 7 October 2019 (UTC)


 * size is a relevant factor, but there are others: mass (as explained above), elasticity, tenacity, etc. A deformable butt would absorbe a significant part of the energy instead of damaging the shooter, while a rigid solid one would not. You also will notice that it is fairly common to not use the butt at all, the recoil being absorbed in the arms, and the same can be done even if the butt is as small as a bullet and in contact with the shooterGem fr (talk) 09:01, 8 October 2019 (UTC)
 * I think the shooter would need to be wearing metal armour to survive such a butt.  Dbfirs  15:26, 8 October 2019 (UTC)
 * This also brings up the Q as to why a more elastic material isn't used for rifle butts, like rubber. Presumably the problem is that the bouncing this would produce would make rapid fire less accurate. SinisterLefty (talk) 15:29, 8 October 2019 (UTC)
 * Shotguns, which can have a heavier recoil because of the heavier shot load, often do have a rubber shoulder pad to absorb the impulse.  Db<i style="color: #4fc;">f</i><i style="color: #6f6;">i</i><i style="color: #4e4;">r</i><i style="color: #4a4">s</i>  15:32, 8 October 2019 (UTC)
 * recoil is known to reduce accuracy in rapid firing (or even just for the second shot), so anything reducing recoil is good. OTOH, a flexible butt will reduce accuracy for even the first shot. All in all, they went for solid. Gem fr (talk) 12:49, 9 October 2019 (UTC)


 * Note that in case of deformation of the butt mentioned above, plasticity would be very undesirable, because you'd need a new butt after shooting.
 * Elasticity is a good thing in this, as it spreads out the transferred impulse over a longer time (i.e. lower forces). Elastic deformation does not absorb energy permanently (it's like a spring, shortening during load and returning to the original length afterwards).
 * Stocks of rifles were (less sure of current rifles) often made of wood, which is far less stiff than steel with a Young's modulus of 11 GPa instead of 210 GPa, while rubbers are between 0.1 and 1 GPa, which is probably too flexible for aiming purposes. Rmvandijk (talk) 07:53, 9 October 2019 (UTC)

No It's not C est moi anton Thanks Anton 81.131.40.58 (talk) 15:06, 8 October 2019 (UTC)