789 reputation
413
bio website henning.makholm.net
location Copenhagen, Denmark
age 41
visits member for 2 years, 11 months
seen yesterday

Apr
30
answered Is there a huge difference between centrifugal and centripetal forces?
Apr
7
comment Drying clothes with the sun's heat, without any air
When you say that water will freeze (and stay frozen, I presume) below 200 K, do you mean that its vapor pressure actually drops to 0 exactly. or merely that sublimation becomes unmeasurably slow even in a vacuum below the theoretical vapor pressure?
Apr
1
comment Is it possible to shoot bullets in space or would the recoil of the gun be too strong?
Even if the gun is floating alone in space when fired, it still has a lot more mass than the bullet, so most of the gas pressure will still go to accelerate the bullet even if the gun itself accelerates slightly backwards during the same time. "Much smaller" is definitely not right.
Mar
15
comment Is it possible to watch the same distant star's supernova event twice?
@Hagen: I'm not denying the facts, merely wondering what the explanation for those facts looks like in the photon-shell picture,
Mar
15
comment Is it possible to watch the same distant star's supernova event twice?
This does raise the interesting question of what shape the photon shell has after it passed the gravitaional lens. Apparently it gets "folded" such that it passes through us several times. But does it stay connected after folding? Are some of our observations because a "reversed" part of the photon shell sweeps past us, so we end up back "on the outside" of the deformed shell, topologically speaking? And how is that consistent with seeing an even number of images?
Mar
14
answered How can shadows ever be sharp?
Feb
23
comment Why is energy not an SI base unit?
Even more accurately: the CGPM decided on unit definitions; the BIPM is tasked with the practical implementation and dissipation of those decisions.
Jan
29
revised Will we ever be able to view the past?
ref Asimov
Jan
29
answered Will we ever be able to view the past?
Jan
19
comment Elevator cabin problem
Well, we can either analyze the situation in Newtonian mechanics, or in (the slow-speed limit of) GR. In the first case gravity is a real force, the lamp really accelerates, the person on the ground is an inertial frame, and the falling elevator is not an inertial frame. On the other hand, in GR all that changes: The elevator is an inertial frame, the lamp is at rest in it, and there is no force of gravity, except for a correction term that the guy on the ground uses to adjust for the fact that his frame is not inertial. Each perspective is valid under its own assumptions.
Jan
17
accepted Why do turbine engines work?
Jan
16
awarded  Nice Question
Jan
9
comment Why are grams usually only expressed as milligrams, grams or kilograms?
@quetzalcoatl: However, when a kiloton goes boom, it is a (non-SI) unit of energy, conventionally equal to one teracalorie.
Dec
5
awarded  Notable Question
Oct
30
comment Fluid flow through an orifice
That doesn't seem to be a matter of mathematics. (You'll need to know at least the viscosity of the fluid, too).
Oct
19
comment How much of the Earth would a spoonful of the Sun scorch?
Actually the temperature of the solar surface (photosphere) is about 6000 K, whereas light bulb filaments are about 3000 K. The fact that light bulbs appears to shine white (at night) shows how well the human vision system white-balance corrects, not that the temperatures are the same. (If you turn on an incandescent lamp in full daylight it will look visibly yellowish).
Oct
18
revised Can a box on a frictionless rotating table rotate?
edited body
Oct
18
answered Can a box on a frictionless rotating table rotate?
Jul
10
comment Special relativity and electromagnets
@PhilFrost: I think a more complex analysis is necessary to explain Faraday's law -- a naive application seem to lead to an effect of the wrong sign. However, one also has to take into account that changes in the electic field propagate with finite speed, and that the electrostatic repulsion from a moving charge in the direction of its movement is less than that of a stationary charge. (In the transverse direction it is the same). This is not part of Coulomb's law but has to be derived using relativity -- it's the only way for things to fit together in a consistent way mathematically.
Jul
10
comment Special relativity and electromagnets
@PhilFrost: From the cat's frame, some electrons are removed from the wire during the transition -- because the cat sees electrons begin leaving the front end of the wire before new electrons begin moving into its back end. (Of course it is an impossibly idealized assumption that the current starts instantaneously from the wire's rest frame, but the net outcome is the same for less sharp transitions).