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Since space and time are relative shouldn't matter and energy be also?

  1. Mar 15, 2012 #1
    Its known that the speed of light is constant...(if you emit a light Eastward while your physically travelling East c will still be the same as if you were still) but also, through an experiment i am imagining, the speed of matter should be relative as well...(if your travelling Eastward and you shoot a bullet to the East it will travel at the same speed as a shooter facing East standing still would). But HOW can matter have the same properties as energy is one has mass and the other not? could matter=energy/frequency?
  2. jcsd
  3. Mar 16, 2012 #2
    This is not correct. If a person fires a bullet while moving with respect to some stationary observer, the bullet will travel faster with respect to the stationary observer than it would have if the shooter was standing still.
  4. Mar 16, 2012 #3
    If you replace 'bullet' with 'photon', then you'ld be correct.
  5. Mar 16, 2012 #4
    but what would the mathematics behind this be? would the kinetic force of the car be ADDED to the velocity of the bullet?
    if the bullet is already in motion, as per the moving car, wouldn't the thrust of the gunpowder need to be greater to overcome the *inertia?* of the bullet?

    ps I'm probably not making a lot of sense since I only know a little bit about a lot of stuff, but please bear with me :)
  6. Mar 16, 2012 #5
    From a Newtonian standpoint, you just add the velocity of car to that of the bullet with respect to the shooter in the car. If a car is moving at velocity vcar and a passenger fires a bullet that travels at velocity u, then the velocity of the bullet with respect to a stationary observer is just:

    vbullet = vcar + u

    In special relativity, "adding" velocities isn't quite this simple, but it's the same general principle.

    Nope, for the same reason you don't fly backwards at over 300 mph when you stand up in an airplane.
  7. Mar 17, 2012 #6


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    Responding to the original post: yes, energy is relative. The kinetic energy of a bullet is 0 in the frame of reference of the bullet. It's much larger in the frame of a person who has shot it. As far as matter, that depends on how you define matter. The rest mass doesn't change with reference frame, but the relativistic mass does change.

    What doesn't change is the length of the energy-momentum 4-vector. This is the rest mass for an atomic particle, but includes internal energies for a composite system.
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