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Does size and scale affect speed

  1. Feb 23, 2012 #1
    Hello I am newto this and maybe it is an easy answer. Here is my question. When I travel in my car at 60 mph. I travel as a whole with the car at 60 mph. But at an atomic size and scale dont the atoms that make up everything really travel faster? Because at that scale wouldnt it be much farther?
  2. jcsd
  3. Feb 23, 2012 #2


    Staff: Mentor

    I think you're confusing atomic motion with the car motion.

    The car goes at 60mph and you're sitting in the car going 60mph. The grain of sand sitting in your pocket is going at 60mph and you can take this analogy down to the atomic level and beyond. Everything in the car is moving with the car at 60mph.

    Because the things around you are at nominal temperatures then that means that they contain some heat energy. Heat energy is basically the movement of the atoms and molecules moving back and forth in all directions, bumping into each other, flexing chemical bonds... The speed of motion at the atomic level is very much faster than the cars speed but the atoms are constantly changing direction so the net displacement over time is zero.

    At the macro level the things in the car are all moving in the same general direction as the car at 60mph
  4. Feb 23, 2012 #3
    Thank you and I do under stand. But I just cant stop thinking if I could be smaller and still be traveling at the larger speed wouldnt I be going faster. It just got me thinking. Light has mass right because itis affected by gravity. And light is made up of photons right? So if something had less mass and was smaller than light couldnt it go faster.That would not change the speed of light or that nothing could go faster just that nothing with that mass could go faster. Im sorry if it sounds silly just trying to wrap my brain around it. Thank you for your help.
  5. Feb 23, 2012 #4


    Staff: Mentor

    Light is made up of photons. Photons have ZERO rest mass that means if you could stop them then they would have ZERO measurable mass. All photons travel at the speed of light. Photons have mass due to the kinetic energy of their motion. Relativity theory says light speed is the fastest any particle of matter could ever go. As a particle begins to approach light speed its energy requirements increase to infinity ie there's not enough energy in the universe to make it go faster. It will never go faster than light speed though.

    If you want to read more about it there are many good sources online. One such is the book on Conceptual Physics Chapter 4 by Brian Crowell:
  6. Feb 23, 2012 #5
    I guess the key word is zero measurable mass but if you put a bunch together in a beam of light it has a small amout of mass gravity can affect thank you for the book referance and who knows maybe one day we will be able to mesure something that small it was not that long ago we could not see the atom thanks again and there is so much to learn it is alot to wrap my mind around
  7. Feb 23, 2012 #6
    Im sorry im still stuck on the term measurable mass just because we cant measure it that means its not there ? But if you put enough together you can there mut be mass there. My scale at home canrt mesure a grain of sand but if i put a bag of them on there i can. Does photons have mass break the laws of physics? I guess I am in over my head here lol it is fun to try to understand though.
  8. Feb 24, 2012 #7
    Hi Broadhead! You said:
    What jedishrfu said above your post is correct: "Photons have ZERO rest mass that means if you could stop them then they would have ZERO measurable mass." (my bolding)

    That is according to theory and experiment, and that what's most likely to be correct today. We can measure the mass of the photon, i.e. measuring what it maximally could be (upper limit) if it's not zero. If it's not zero, it's very, very small (see below).
    No and "yes" :smile:. It's not like we would have to rework all of physics, but some things would have to be changed and rethought. It wouldn't be as serious as let's say breaking the energy conservation law or accelerating a mass above the light speed. This section in Wikipedia describes in short both experiments on the mass and some of the implications if the mass would be nonzero. Here is some more on the issue.
  9. Feb 24, 2012 #8
    Broadhead, I forgot to reply to this:
    That is sort of correct. Without going into details, it's more like the light can be thought of having mass (it has energy).
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