Remember that energy is never created nor destroyed but simply changes form. So, besides some of the energy being used to cause the two masses to stick together in a perfectly inelastic collisions what else could account for some of the lose of the mechanical energy

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?