Momentum is conserved unless an external force (e.g. Friction) acts; Momentum = mass times velocity and since velocity is a vector; we have to remember the directions for momentum calculations too. “Right-is-positive” often works well.
Bombardment of certain stable elements with proton radiation to make them into radioactive isotopes that usually emit positrons. The wrong mix of protons and neutrons means that it will be unstable; excess protons in a nucleus cause beta-plus radiation.
Positrons are Anti-matter; when they are emitted as beta-plus radiation, they will very soon meet some matter; electrons orbit around every atom and attract positrons.
Electron plus positron leads to Annihilation; all the mass is converted to energy;
‘E’ = ‘m’ ‘c’ squared shows a small amount of matter can make a lot of energy (‘c’ squared = 9 x 1016); the resultant energy is carried away as gamma radiation.
Meeting of a positron with an electron is an example of momentum conservation and mass/energy conservation:
Mass and energy can be…