A pion has a rest energy of 135 mev. it decays into two gamma ray photons, bursts of electromagnetic radiation that travel at the speed of light. a pion moving through the laboratory at v = 0.98c decays into two gamma ray photons of equal energies, making equal angles θ with the original direction of motion. find the angle θ and the energies of the two gamma ray photons.

As you are trying to move a heavy box of mass m, you realize that it is too heavy for you to lift by yourself. there is no one around to , so you attach an ideal pulley to the box and a massless rope to the ceiling, which you wrap around the pulley. you pull up on the rope to lift the box. use g for the magnitude of the acceleration due to gravity and neglect friction forces. once you have pulled hard enough to start the box moving upward, what is the magnitude f of the upward force you must apply to the rope to start raising the box with constant velocity? express the magnitude of the force in terms of m, the mass of the box.