Physics Help Forum Does momentum "get transferred" during collisions?

May 21st 2018, 02:32 AM   #11
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 Originally Posted by justme I want to get the words right.
OK so you do not want to have a discussion with me.

I will leave you to enjoy the tender mercies of PMB.

 May 21st 2018, 05:08 AM #12 Senior Member     Join Date: Jun 2016 Location: England Posts: 933 What are Mass & Energy? Physics is perhaps surprisingly bad at providing answers to these sort of questions. Similarly poorly defined concepts include Distance and Time! One tends to get into circular arguments: Energy is the quantity that satisfies these equations; these equations define energy... Physicists might argue that "what" these quantities "are" is a meta-physical question. Physics concentrates only on what these quantities "do". __________________ ~\o/~ Last edited by Woody; May 21st 2018 at 12:36 PM.
 May 21st 2018, 06:21 AM #13 Senior Member   Join Date: May 2014 Location: Poole, UK Posts: 132 Mass, or should I say rest mass, is resistance to change in motion. It's easier to stop a skateboard than a car, because the latter has more mass. It's also a measure of a body's energy-content, as Einstein said in his 1905 paper Does the inertia of a body depend upon its energy content? As to why, see a paper by Martin van der Mark and Gert (not the Nobel) ‘t Hooft called Light is Heavy. It talks about a balance scale, and explains why a box containing hot gas is heavier than a box containing cold gas. Imagine something like a gedanken spring steel box in space full of gedanken spring steel bullets ricocheting around inside. When you push the box, you have to push against the bullets bouncing off the side you’re pushing against. When I tap my magic wand and make those bullets move faster, you find it harder to get the box moving. So the box has more mass. Van der Mark and ‘t Hooft go on to replace the gas with light, whereupon the same principle applies. When you push the box, you have to push against the photons bouncing off the side you’re pushing against. When I tap my magic wand and increase the photon frequency, you find it harder to get the box moving. So the box has more mass. Note that the mass of a body is a measure of its energy-content. Energy is a real thing. Matter is made of it. As for what energy is, that's more difficult to explain, because energy is fundamental. I can tell you that your sideboard is made of wood, and I can tell you that wood is made of carbon atoms plus other atoms, and that a carbon atom is made up of electrons plus other particles. Then I can tell you that we can make electrons and positrons out of photons in pair production. I can also tell you what a photon is. It's an E=hf transverse wave "soliton" in space that propagates at c. In addition I can tell you that Einstein considered it to be energy. However there's also neutrinos, and black holes which are comprised of energy, and they aren't photons. So in the end I'm reduced to saying energy is spatial stress-energy-momentum. Last edited by Farsight; May 21st 2018 at 06:23 AM.
 May 21st 2018, 10:06 AM #14 Physics Team   Join Date: Apr 2009 Location: Boston's North Shore Posts: 1,576 Energy is not a real thing. You can't pick, it up. It can't be detected. You can't see, feel or hear it. Energy is merely a booking concept. Read the quotes in this page and you may understand.
 May 21st 2018, 03:59 PM #15 Senior Member   Join Date: May 2014 Location: Poole, UK Posts: 132 It isn't just some bookkeeping thing. Read Einstein's E=mc˛ paper. Note this line: "The mass of a body is a measure of its energy-content". Also think of an ocean wave. When you take some of the energy out of that wave, the wave is diminished. When you take all the energy out of that wave, the wave isn't there any more. It's the same for a photon. And note that we can make matter out of photons in gamma-gamma pair production. I don't know where you've got this bookkeeping idea from, but it doesn't square with Einstein, or with the hard scientific evidence. It's wrong. Sorry.
May 22nd 2018, 12:51 AM   #16
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Farsight, your material is a farcry from the OP, who seems to be just learning elementary classical Physics.

 Originally Posted by justme Ok so there is no such thing as mass, energy or momentum..they are just "properties" of matter. Can force be called a "property" of matter? Can torque be called a "property" of matter? Is matter a "thing"? Thanks, I want to get the words right.

In particular photons are not 'matter'.

 May 23rd 2018, 06:04 AM #17 Senior Member   Join Date: May 2014 Location: Poole, UK Posts: 132 Studiot: I responded to where the thread had gone. Yes, I know photons are not matter. But when you trap a massless photon in a gedanken mirror-box, it's harder to move as a result. It's mass is increased. Then when you open the box and the photon flies out, the mirror-box is a radiating body that loses mass. It's all pretty simple stuff, see https://arxiv.org/abs/1508.06478.
May 23rd 2018, 12:04 PM   #18
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 Originally Posted by studiot Farsight, your material is a farcry from the OP, who seems to be just learning elementary classical Physics. In particular photons are not 'matter'.
That's subject to opinion. Some authors (Feynman, Guth, etc.) sometimes use relativistic mass and as such a photon (or a directed beam of radiation) has mass and as such its matter.

Hans C. Ohanian refers to the EM field as the fourth state of matter.

 May 24th 2018, 05:44 AM #19 Senior Member   Join Date: May 2014 Location: Poole, UK Posts: 132 Hans C Ohanian also wrote What is Spin? which I think is a great paper.
 May 25th 2018, 10:37 AM #20 Senior Member   Join Date: Jun 2010 Location: NC Posts: 417 Some Example Calculations... Hi justme... Just for your reading, here are three math solutions involving momentum. Jet-Ski Static Pull | THERMO Spoken Here! Springfield Rifle Musket (1861) | THERMO Spoken Here! P-51 Mustang | THERMO Spoken Here! Good luck... JP

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