# Where does the kinetic energy of an object annihilated in a matter antimatter reaction go?

A bullet is moving at $$0.99 c$$. Now we annihilate it by having it collide with an equal mass of antimatter. (assume instant and perfect reaction) This will turn both bullet and antimatter into photons.

But where did the substantial ammount ot kinetic energy carried by the bullet go? Momentum needs to be conserved. Have I just created a radiation beam moving along the vector of the bullet?

• Depends on what it annihilates into. If it's photons, the photons will be higher in energy than they would have been if the bullet hadn't been moving. – knzhou Apr 21 '19 at 13:02

The mass would be converted to energy through mass-energy equivalence, and additional kinetic energy from before the collision would also have to be conserved. Usually the velocities are so small relative to the speed of light that we ignore the kinetic energy as most of the photon energy is from mass annihilation. If the kinetic energy was significant it would be conserved in the created photons by increasing their frequency and thus momentum. The energy of a photon is known by the equation $$E=hf$$ with h being Planck's constant and f being the frequency.