Prove that conservation of four-momentum forbids a reaction in which an electron and positron annihilate and produce a single photon (y-ray). prove that the production of two photons is not forbidden.

current and voltage can both be different in different parts of a circuit so it matters where you place the ammeter and voltmete

the moving of the box is a result of centripetal force. think of that  as a car or a truck in this case creates a turn, the force of friction performing upon the turned wheels of the car runs centripetal force obligatory for circular motion for the things inside it.