Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Does a bullet exiting a gun at speeds approaching the velocity of light?

Learn more physics!

Most recent answer: 03/30/2016
Q:
when we are going in a light speed at that time we shoot a bullet will that bullet come out or not?
- kovarthanan (age 18)
chennai,tamilnadu,india
A:

By far the most important point is that it is meaningless to ask if we're moving. That's purely a matter of point of view, a choice of reference frames.

We can't go at light speed in any frame, only zero mass particles, like photons can do that.  

From our own point of view, we're at rest so we'd see the bullet go away from us at its usual speed, say u, (so yes, it would go out).

From the point of view of somebody who says we're moving at velocity v almost as big as c,  the bullet would be going at speed s just a little bit more than ,    s = (u +v) / (1 +uv/c^2 ).  (This calculation assumes that the bullet is fired along the direction in which the observer says we're moving.) This is only a tiny bit more than v, so that person at rest would see it take a long time for the bullet to leave the gun. 

In general, how fast that observer says the bullet leaves the gun depends on whether the bullet is fired along the direction of your motion (in their frame) or at right angles to it, or in between. They see the bullet speed u reduced by a factor (1-v2/c2)1/2 for motion at right angles to the direction of your motion, just because they think that your clocks are running slow. There's another factor of  (1-v2/c2)1/2 for bullets fired along the direction of your motion, because they see your meter-sticks shrunk along that direction.

For more discussion of this see https://van.physics.illinois.edu/qa/listing.php?id=16557, or http://van.physics.illinois.edu/QA/listing.php?id=16290

 


(published on 03/08/2016)

Follow-Up #1: error correction

Q:
Physics Van: You said "This is only a tiny bit more than u..." I guess you meant "This is only a tiny bit more than v...".
- Mehran (age 65)
Miami
A:

Thanks Mehran, my bad. Fixed. Mike W.


(published on 03/30/2016)

Follow-up on this answer.