Kinetic to heat (building a spaceship)
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From: jbriggs444 on 11 Jun 2010 08:33 On Jun 10, 6:27 pm, gb <gb6...(a)yahoo.com> wrote: > On Jun 10, 12:55 pm, Sam Wormley <sworml...(a)gmail.com> wrote: > > > > > > > On 6/10/10 2:46 PM, gb wrote: > > > > Usually people want to avoid friction, I want to create one. > > > > The idea is that there is a spaceship in space. You throw something, a > > > weight from the front of the spaceship to the back of the spaceship, > > > then the spaceship moves. > > > > But when the weight reaches the end of the spaceship, the spaceship > > > stops in space. > > > > Now what if we convert kinetic energy into heat? Heat is this thing > > > which can be lost as friction between the weight leaving the top of > > > the spaceship and arriving to the end. > > > > What is launched at the top can be some chemical, which transforms > > > from kinetic motion to where the energy of motion is lost as heat and > > > friction. > > > > Heat can absorb energy of motion where particles lose energy by > > > colliding with one another and they build heat. > > > > This spaceship, or flying saucer would pulse energy this way > > > internally without letting matter escape, would use a nuclear reactor, > > > and is a master of cruel friction. > > > > How could such a spaceship be built technologically where kinetic > > > energy is transferred into heat, producing valid motion in physics by > > > such a spaceship? > > > A spaceship can be thought of as a closed system with constant > > momentum. Unless acted upon be an external force or unless the > > spaceship sheds matter or radiation, nothing much happens. > > dp/dt = 0 > > But what happens to kinetic energy when friction transfers energy > of motion to heat? Heat is not the same energy as kinetic energy. > > It starts out as motion which dp/dt=0 refers to, but transfers into > heat. The heat is something that can be lost. > > If the spaceship starts out with moving something under it, > then transfers that something from weight to heat, the kinetic > momentum is gone. > > The spaceship pushes itself against something which turns into > heat a moment later. > > But thanks for the info.- Hide quoted text - > > - Show quoted text - The root problem with this scheme is that ignoring momentum does not make it go away. Say you're up at the front of the space craft and you launch a bowling ball backwards. The space craft goes forward as the bowling ball toes backward. So far, so good. Now suppose that you catch this bowling ball in some kind of tube lined with sandpaper so that you can turn its kinetic energy [relative to the ship] into heat. That's fine. You can do that. You can reduce its kinetic energy to zero. The principle of conservation of energy assures you that this energy is not lost. It will appear as heat in the bowling ball and the sandpaper. While the bowling ball is slowing and and while you are carefully considering the virtues of the principle of conservation of energy, the principle of conservation of momentum still applies. As the bowling ball loses backward momentum in its interaction with the sandpaper, the ship is guaranteed to lose equal and opposite forwad momentum. This is Newton's third law in action -- when the sandpaper acts on the bowling ball, the bowling ball also acts on the sandpaper. By contrast, propulsion by radiation of heat energy works. A problem with propulsion by radiation of heat energy is that the ratio of energy used to propulsion produced is very low. There can be a lot of energy in the exhaust stream but very little momentum. We all know this intuitively. Nobody braces themselves against the recoil of a shining flashlight. The efficiency of a floodlight as a propulsion device is vanishingly low. The ratio of energy used to momentum harvested is given by: 1/2 mv^2 / mv = 1/2 v. The higher the exhaust velocity, the lower the efficiency. As relativity kicks in at higher speeds, this ends up being off by a factor of two. pc / p = c = v The ideal case (in terms of energy budget) for a rocket design is when you're throwing as much mass out the back as you possibly can at as high a velocity as you can muster without exceeding your energy supply. That means that should throw out all the burned fuel at least. That means that the optimal exhaust velocity is bounded above based on the energy/mass ratio of the fuel supply. If you are using a matter-antimatter drive, _then_ using a light-speed exhaust stream is appropriate. With chemical energy you're talking about something that's many orders of magnitude slower (in the very rough neighborhood of Mach 10).
From: gb on 11 Jun 2010 18:09 On Jun 10, 4:17 pm, Sam Wormley <sworml...(a)gmail.com> wrote: > On 6/10/10 5:27 PM, gb wrote: > > > > > > > On Jun 10, 12:55 pm, Sam Wormley<sworml...(a)gmail.com>  wrote: > >> On 6/10/10 2:46 PM, gb wrote: > > >>> Usually people want to avoid friction, I want to create one. > > >>> The idea is that there is a spaceship in space. You throw something, a > >>> weight from the front of the spaceship to the back of the spaceship, > >>> then the spaceship moves. > > >>> But when the weight reaches the end of the spaceship, the spaceship > >>> stops in space. > > >>> Now what if we convert kinetic energy into heat? Heat is this thing > >>> which can be lost as friction between the weight leaving the top of > >>> the spaceship and arriving to the end. > > >>> What is launched at the top can be some chemical, which transforms > >>> from kinetic motion to where the energy of motion is lost as heat and > >>> friction. > > >>> Heat can absorb energy of motion where particles lose energy by > >>> colliding with one another and they build heat. > > >>> This spaceship, or flying saucer would pulse energy this way > >>> internally without letting matter escape, would use a nuclear reactor, > >>> and is a master of cruel friction. > > >>> How could such a spaceship be built technologically where kinetic > >>> energy is transferred into heat, producing valid motion in physics by > >>> such a spaceship? > > >>   A spaceship can be thought of as a closed system with constant > >>   momentum. Unless acted upon be an external force or unless the > >>   spaceship sheds matter or radiation, nothing much happens. > >>   dp/dt = 0 > > > But what happens to kinetic energy when friction transfers energy > > of motion to heat? Heat is not the same energy as kinetic energy. > > > It starts out as motion which dp/dt=0 refers to, but transfers into > > heat. The heat is something that can be lost. > >   If heat (electromagnetic radiation) is radiated in one principle >   direction, then each photon had momentum, p = hν/c = h/λ and one >   could propel (slightly) a spacecraft, as conservation of momentum >   applies. If the radiation is equal in all directions, then the >   dp/dt for the spacecraft as a whole, remains zero. I know that it is unclear if this concept works, but it is based on the opposite concept of perpetual motion machines, bad friction, so it is called stubborn machines. Radiation loss (internally lost energy of momentum) may be interesting research, but may turn out as illegal as perpetual motion machines. Kinetic energy may be impossible to lose away in any form.
From: gb on 11 Jun 2010 18:20 There are ways with pressure to accelerate something, like hot and cold gas. Compressed liquid will immediately turn into gas, and cold will accelerate toward hot. The particles slam on the bottom of the ship stronger from introduced pressure which accelerates the motion of particles. Can use hot and cold difference, or pressure difference. In the end the resulting slam at the bottom of the spaceship produces the stop, which may be a stronger force than what initially was thrown or released downward.
From: Sam Wormley on 11 Jun 2010 21:58 On 6/11/10 5:09 PM, gb wrote: > Radiation loss (internally lost energy of momentum) may be interesting research, > but may turn out as illegal as perpetual motion machines. Kinetic energy may > be impossible to lose away in any form. You have missed a very fundamental finding in physics. Momentum is always conserved! There is no "internally lost energy of momentum" You appear not to know the differences between the concepts of momentum, energy and thermodynamics. Easily remedied with a freshman level physics textbook. Google is your friend.
From: Sam Wormley on 11 Jun 2010 22:01
On 6/11/10 5:20 PM, gb wrote: > There are ways with pressure to accelerate something, like hot and > cold gas. Compressed liquid will immediately turn into gas, and cold > will accelerate toward hot. The particles slam on the bottom of the > ship stronger from introduced pressure which accelerates the motion of > particles. > > Can use hot and cold difference, or pressure difference. In the end > the resulting slam at the bottom of the spaceship produces the stop, > which may be a stronger force than what initially was thrown or > released downward. You cannot fool mother nature. Momentum is conserved in closed systems. http://en.wikipedia.org/wiki/Momentum |