Wikipedia:Reference desk/Archives/Science/2015 July 3

= July 3 =

Camera with built in selfie stick
Why don't camera manufacturer create a point and shoot camera with a built in selfie stick and a shutter button on the selfie stick itself? 220.239.43.253 (talk) 01:09, 3 July 2015 (UTC)


 * (1) How do you know they don't? (2) Why don't you contact some major manufacturers and present them with your idea? ←Baseball Bugs What's up, Doc? carrots→ 01:51, 3 July 2015 (UTC)


 * I thought most cameras had the standard tripod thread (not on cell phones). If it doesn't make it smaller, it's prbably not cost effective.  A lot of feature decisions are based on differentiation and what people will pay for differentiation.  If "built-in selfie stick" is a cool feature but not something people will pay more or at the same price allow market share growth, it won't get added because it adds to the BOM without an ROI.  3D TV's and 3D cameras are products that had higher BOM with little ROI and they tend to disappear from the market.  --DHeyward (talk) 03:14, 3 July 2015 (UTC)


 * BOM = bill of materials (effectively manufacturing cost), ROI = return on investment (effectively profit). Smurrayinchester 08:23, 3 July 2015 (UTC)


 * Are selfie sticks used for cameras? I know they CAN be used with a camera, but the only people I see that use a selfie stick are people who use their cell phone as a camera. 199.15.144.250 (talk) 16:36, 3 July 2015 (UTC)


 * We do have an article on selfie stick. Apparently there are some specifically designed to use with non-phone cameras. I don't know if anyone has bothered (yet) to gather statistics on usage with cameras that have phones compared to cameras that do not have phones. SemanticMantis (talk) 17:12, 3 July 2015 (UTC)


 * One caveat could be that non-phone cameras tend to be heavier than phone cameras, which could make the leverage more difficult. ←Baseball Bugs What's up, Doc? carrots→ 22:08, 3 July 2015 (UTC)


 * I wouldn't suggest that the selfie-stick be truly built in, as that would add weight and bulk when you don't need it. They could make one as an accessory that has a shutter button, maybe using Bluetooth to send that signal to the camera/cell phone.  I'd use the standard tripod connection.  Hopefully the camera/cell phone and selfie-stick could also be  replaced independently of each other. StuRat (talk) 22:26, 3 July 2015 (UTC)
 * Googling "digital camera selfie stick" indicates that there are selfie sticks for non-iphone cameras. ←Baseball Bugs What's up, Doc? carrots→ 23:14, 3 July 2015 (UTC)

Measuring displacement, velocity, acceleration, jerk, etc.
Hi, we can measure the displacement of a body, we can also measure its velocity, but can we measure the derivatives of displacement wrt time of any order? Thanx! 5.29.9.245 (talk) 10:03, 3 July 2015 (UTC)


 * Displacement and velocity can be measured but only after we have defined the reference frame in which the measurements will be taken. This dependence on reference frame means these two measurements are always arbitrary - a person using a different reference frame will achieve different measurements. In contrast, force, mass and acceleration are not dependent on our choice of reference frame (at least in Newtonian mechanics).  Different persons using different reference frames for the measurement of these things will all measure the same acceleration (and force and mass).
 * For example, a traveler in space or in orbit can turn on an accelerometer and read his acceleration (by comparison with zero acceleration he experienced when calibrating the accelerometers) but there is no similar device that will display displacement or velocity. Dolphin  ( t ) 10:58, 3 July 2015 (UTC)


 * If we fix a reference frame (and we don't know whether or not it is an inertial frame of reference) then we can measure the (vector) displacement of an object in that reference frame at various points in time. If we make two measurements in a small enough time interval then we can *approximate* the instantaneous velocity of the object, and if we make more measurements in a small enough time interval then we can *approximate* its instantaneous acceleration, jerk etc. But I don't know of any way to actually *measure* instantaneous velocity, acceleration, jerk etc in a general reference frame. We can only use F = ma to measure acceleration if (a) we know the mass of the object - which requires a reference mass unless maybe our object is a fundamental particle - and (b) we know that our reference frame is inertial. Gandalf61 (talk) 11:24, 3 July 2015 (UTC)


 * See accelerometer. However, the acceleration due to gravity is included in that measurement, so you would need to do some vector math to remove that component, if near a massive body (like Earth).  StuRat (talk) 16:04, 3 July 2015 (UTC)


 * ... but to calibrate the accelerometer you need some independent means of measuring acceleration. Gandalf61 (talk) 16:19, 3 July 2015 (UTC)


 * I don't really understand this comment, or your previous one. One second is 9192631770 ticks of a cesium clock, without reference to any standard timepiece in France. The kilogram is still a physical object, but only because of practical engineering difficulties. One m/s² could likewise be defined by the behavior of a particular kind of solid-state accelerometer, practical difficulties aside. -- BenRG (talk) 23:02, 3 July 2015 (UTC)


 * Perhaps for acceleration he means that the acceleration due to gravity isn't always exactly 9.81 m/s2, as it varies a tiny amount by location (on the surface of the Earth) and elevation (and of course quite a bit more in space). So, for extremely accurate readings, you would want to calibrate it for your current location.  The direction of acceleration would also be quite critical, so you would need a way to accurately measure the angle, relative to the pull of gravity in that location. StuRat (talk) 23:39, 3 July 2015 (UTC)


 * Indeed. An accelerometer does not measure local acceleration - it measures displacement which it translates into force, and then it translates that force into acceleration. To show local acceleration the accelerometer has to be calibrated for a specific location to compensate for gravity and other non-inertial forces. Take an accelerometer calibrated for the Earth's surface and put it on the surface of the Moon and it will tell you that it is accelerating downwards at 5/6 g because it now measures a downwards force that is less than when it was calibrated. And to calibrate an accelerometer correctly you have to ensure it has zero local acceleration. And you cannot measure the local acceleration of an object directly, you can only estimate it from multiple measurements of displacement. Gandalf61 (talk) 08:27, 4 July 2015 (UTC)


 * If clocks measure local time, then accelerometers measure local acceleration. The reason they show an upward acceleration when sitting on a table is that the table is accelerating upward. There is only one correct (generally covariant) concept of acceleration in a general relativistic world, and that's what accelerometers measure.
 * The important thing is that "velocitometers" and "positionometers" don't exist, at least not as self-contained memoryless devices like accelerometers. The original question has it backwards in some sense. You can figure out an approximate spacetime velocity and position using inertial navigation, but you do that by integrating the acceleration, and you have to supply the boundary conditions (initial position and velocity) from outside because they're not intrinsically measurable like acceleration is. Jerk and higher derivatives can be calculated internally as time derivatives of the acceleration (with respect to an internal cesium clock). Velocity and position are the only problematic ones. -- BenRG (talk) 10:03, 4 July 2015 (UTC)


 * Obviously the OP is asking about measuring position, velocity, acceleration etc. relative to a local co-ordinate system which will, in general, be non-inertial. So an answer that tells them that the table that they think is stationary has a proper acceleration of approximately 9.8 ms-2 upwards and this is the only covariant way to definie acceleration is factually correct but completely useless as an answer to their question. Gandalf61 (talk) 14:54, 4 July 2015 (UTC)
 * I can think of at least a couple of exceptions where we do measure position and velocity absolutely, and not as the integral of acceleration with respect to time. For example, an ultrasonic pinger (ranger), a gray coded position or rotary encoder, a Doppler RADAR, an altimeter, and a vertical speed indicator each measure absolute linear or angular position, or absolute velocity, directly - and not as a time derivative or as an integral of anything.  These machines only work within certain limits, and depend on certain assumptions about natural physical properties; some of these properties are hardly universally true and it would be foolish to think that the measurements are valid outside of the intended range of operation; but the same can be said of any machine that measures any physical property - even if we are trying to think in a way that is consistent with general relativity!  Atomic clocks are only accurate if we can disavow thermal noise; accelerometers usually do measure displacement, either by assuming some material property that defines a spring constant or electrostatic relationship; and so on.  It's quite easy to say that a General Relativistic model is absolved from all such practical details... but try to build a device that is relativistically correct and has no dependence on any empirical assumption at all!  It can't be done.  Even if we look at famous tests of measurement of General Relativity - like Gravity Probe A - the precise measurements did not take place in a Platonic-idealistic world.  When we test measurements of general relativity, we are still constrained by imperfections and assumptions that simply do not apply everywhere in the universe, and sometimes, our pure physics is even tainted by practical engineering details!  Nimur (talk) 04:04, 6 July 2015 (UTC)
 * I'm not quite sure what type of question this is. Is it a tool question about measurement (i.e. do we have accelerometers, do we have displacement meters like the Plimsoll line on ships that calculate in time)?  Or is it a relativity question regarding reference frames?  Or is it a quantum mechanics question regarding Heisenberg uncertainty?  The answer depends on which of the three (or possibly which combination) the poster is seeking.  Displacement of a ship, for example is a pretty simple static calculation, but when under power and acceleration, displacement is compoinded by those forces in addition to things that affect mass per unit volume of water such as salinity and temperature.  --DHeyward (talk) 00:58, 4 July 2015 (UTC)

I am familiar with two types of accelerometer. One uses a strain gauge to measure the deflection of a cantilever with a mass on the end of it. As such it measures down to DC, hence measures the acceleration due to gravity, even if the accelerometer is stationary. The other uses a piezo electric force gauge to measure the forces experienced by a small mass. These are typically used down to 3, 1 or 0.3 Hz (this is an instrumentation decision). Within those frequency limits they both measure the absolute acceleration, ie they are operating in an inertial reference frame.Greglocock (talk) 02:57, 5 July 2015 (UTC)

The fourth derivative of displacement is called jounce, or snap. I learned "jerk" and "jounce" at the same time, but when I mentioned it to someone, they'd heard of jerk but not jounce. So, in case anyone didn't know, there it is. Roches (talk) 08:27, 7 July 2015 (UTC)

Egg Hatching
How long should it take for Dove eggs to hatch? — Preceding unsigned comment added by 2601:581:8400:9D36:CA2A:14FF:FE3F:3730 (talk) 15:34, 3 July 2015 (UTC)


 * It will depend on the species, you can easily google for results and add the species name for more specific info. But if your Dove Eggs hatch, you should film it and call the local news. μηδείς (talk) 11:47 am, Today (UTC−4)


 * lists 15 days, but it might vary by species/subspecies. StuRat (talk) 15:53, 3 July 2015 (UTC)
 * It won't very that much though. The shortest dove incubation I can find is 12+/- 0.8 days, here, and the common mourning dove takes 13-14 days . Interestingly, the little desert diamond doves in the first link are nearing a fairly absolute limit of about 11 days for any bird. This paper discusses how many birds were reported to have 9-10 day incubation times, but those were probably due to bad methodologies, and not accurate measurements of real incubation. As of 1953, there were no records of any incubation under 11 days. I also couldn't find any good records of a dove species with a 16 day incubation period.  SemanticMantis (talk) 16:28, 3 July 2015 (UTC)


 * It won't "very" at all, but it might "vary" a bit. :-) StuRat (talk) 15:04, 7 July 2015 (UTC)