A320.9 ng sample of an unknown radioactive substance was placed in storage and its mass measured periodically. after 47 days the amount of radioactive substance had decreased to 40.11 ng. how many half‑lives of the unknown radioactive substance have occurred?

Air initially at 0.75 bar, 1000 k, and occupying a volume of 0.12 m^3 undergoes two processes. process 1-2: the air is compressed isothermally until the volume is halved. process 2-3: the air undergoes a constant pressure process until the volume is halved again. assume ideal gas behavior. a) determine the mass of the air, in kg. b) the work and the heat transfer for each of the two processes, in kj. (100 kj = 1 bar . m^3)

Who was the first scientist to measure speed as distance over time?

Which sound characteristic is not affected by the relative motion of an object