A hydrogen atom initially in the n = 2 level emits a photon and ends up in the ground state. (a) What is the energy of the emitted photon? (I got 10.2 eV) (b) If this atom then absorbs a second photon and returns to the n = 2 state, what must the energy of this photon be

An astronaut in space cannot use a scale or balance to weigh objects because there is no gravity. but she does have devices to measure distance and time accurately. she knows her own mass is 77.4 kg , but she is unsure of the mass of a large gas canister in the airless rocket. when this canister is approaching her at 3.50 m/s , she pushes against it, which slows it down to 1.30 m/s (but does not reverse it) and gives her a speed of 2.60 m/s . what is the mass of the canister?