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 78.4 kg , but she is unsure of the mass of a large gas canister in the airless rocket. When this canister is approaching her at 3.30 m/s , she pushes against it, which slows it down to 1.20 m/s (but does not reverse it) and gives her a speed of 2.40 m/s .?