Jan Hudec
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 May 2 comment How is 6W equivalent to 40W, as claimed by adverts for LED light bulbs? @jamesqf, the monochromatic LEDs are more efficient. But to get white light a luminophore is needed and that reduces the efficiency quite a bit. Apr 26 awarded Nice Answer Apr 25 revised Do all equations have identical units on the left- and right-hand sides? added 513 characters in body Apr 25 answered Do all equations have identical units on the left- and right-hand sides? Apr 25 comment Do all equations have identical units on the left- and right-hand sides? $G$ does not, necessarily, have that unit. E.g. $G \dot= 3.8487\cdot 10^{-6}\frac{\mathrm{ft}^3}{\mathrm{lb}\cdot\mathrm{min}^2}$. Apr 12 comment Why does a ceiling fan start up slowly? @Dubu, it would be more complicated too. The simple construction simply has the torque it needs for the normal operation and the engine runs at the same power from the start, so it spins up and the acceleration decreases as the air resistance builds up. To make it spin up faster, the engine would need higher torque during start than later for normal operation and therefore would additionally need a regulator. Apr 11 comment Can lasers lift objects? With perfect mirror, the laser would lift twice as much, because the reflected light also exerts the force too (the leaving photons also have momentum, that must be taken from the lifted object). Feb 26 comment What's the difference between mechanical and electromagnetic waves? Note that the maximum speed is always speed of light in vacuum. Light is slowed down in other environments and there a particle with nonzero mass can travel as fast or faster than light, generating Cherenkov radiation as a side-effect. Feb 1 comment Coefficient of friction and practical experience of sliding I am comparing laying with sitting on the butt, not standing. Standing is completely irrelevant, for the reasons you mention and also because shoes have much higher friction than jacket (by design). When sitting, the CoM is slightly higher then laying, but low enough to slide without toppling. Feb 1 asked Coefficient of friction and practical experience of sliding Jan 27 comment Backyard experiments to falsify the Flat Earth theory Combining with physics.stackexchange.com/a/26431/6395, this can be improved. Take the unbeliever to the sea shore (no possibly photoshopped pictures) somewhere where they have a watch tower. Wait for a ship to start disappearing below the horizon, then climb to the tower and observe the ship is still visible from there. Jan 18 comment How am I able to stand up and walk down the aisle of a flying passenger jet? You don't even need to consider a plane. You are walking on Earth, which rotates with speed up to 1670 km/h (depending on your latitude), orbiting the Sun at nearly 30 km/s (and orbiting the centre of galaxy, but I don't know what that speed is; probably even larger though). Nov 28 awarded Yearling Nov 20 comment Why does rotation simulate gravity if motion is relative? @Flosculus, because the reference frames rotate relative to each other, the accelerations in them will be different. The differences will imply existence of a centrifugal force for the observer on surface and that implies he is in rotating frame of reference. The observer flying by is not needed; just measuring the centrifugal force at various points on the surface allows determining the axis of rotation and angular speed. Nov 20 comment Why does rotation simulate gravity if motion is relative? @JeremyOlson, every universe has an infinite number of reference frames, because reference frame is just a mental construct. However, you need to attach such reference frame to some object to be able to observe laws of physics in it. Now in a universe that only contained one point mass there would be no way to accelerate it, because momentum must be conserved, and no way to spin it, because angular momentum must also be conserved. For both you need the universe to contain multiple particles and then you have enough objects to measure relative speeds of to detect acceleration. Nov 20 comment Why does rotation simulate gravity if motion is relative? @Flosculus, it implies there are static axis, but any set of static axis is still equivalent. Oct 21 answered Does the force between two point charges change when inertial reference frame changes? Oct 12 comment Why can't helicopters reach mount everest? @AndyM, also note, that it is the stream velocity orthogonal to the structure that matters, so aircraft use swept wings to delay the effects of reaching speed of sound. Some modern propellers are also curved for the same purpose. But helicopters can't simply use curved blades, because the blades must be precisely balanced as they are tensed by the centrifugal force, but some new helicopters do have blades that are swept a bit forward and then back again near the tip to get some benefit of sweep while keeping the centre of mass where it needs to be. Oct 12 comment Why can't helicopters reach mount everest? @AndyM, I did not actually say it is dangerous per se. The problem is that as the flow exceeds speed of sound, the drag raises greatly and the lift drops. So if the rotor blades tips approach speed of sound, they won't be able to accelerate further, may be damaged by the forces and won't deliver more lift anyway. Sep 28 comment If action equals reaction, how is it ever possible to win in martial arts? It also matters where the momentum ends up. Many strikes are not intended to hurt, but to throw the opponent out of balance by transferring more momentum than the opponent can pass on to the ground in that direction.