# Going the speed of light, possibly.

• DrWafflez Jr.
In summary, the speed of sound through a medium is not affected by the speed of the emitter and at relativistic speeds, you cannot add velocities as you usually do. Sound waves have a particular speed in the air and that's the speed they will move at regardless of the speed of the source. Similarly, the velocity of a bullet fired from a moving car can be calculated using the velocity addition formula, which reduces to the expected sum of velocities at slower speeds.

#### DrWafflez Jr.

I have a question, my bestfriends dad said that if we were driving at 70 mph and then honked the horn, the sound would go 70 mph + the speed of sound. So the obvious question, if that is true can we go the speed of light? But sound is the vibrations of atoms? Does that mean the atoms would have to bump into each other at the speed of light? Last but not least can this be accomplished with a gun being shot or something of the sort? Thanks in advance

DrWafflez Jr. said:
So the obvious question, if that is true can we go the speed of light?

No. At relativistic speeds, you can not add velocities like you usually do, because of special relativity. See Velocity-addition formula (Wiki) and Einstein Velocity Addition (HyperPhysics).

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Your friend is incorrect, the speed of sound through a medium is not affected by the speed of the emitter.

DrWafflez Jr. said:
I have a question, my bestfriends dad said that if we were driving at 70 mph and then honked the horn, the sound would go 70 mph + the speed of sound. ... Last but not least can this be accomplished with a gun being shot or something of the sort? Thanks in advance

Honk the horn and the sound will travel at the speed of sound no matter the speed of the source - sound waves have a particular speed in the air and that's the speed the move at.

Fire a bullet forward from a moving car and the speed of the bullet relative to the ground will be given by the velocity addition formula DennisN mentions (##\frac{u+v}{1+uv/c^2}##) where ##u## is the speed of the car relative to the ground and ##v## is the muzzle velocity of the bullet. For cars and bullets and other slow-moving objects (here "slow-moving" means not tens of thousands of miles a second) this formula reduces to the ##u+v## that common sense tells us to expect.