Question

(a) Black holes are invisible. Why? How are they identified?
(b) How do multistage rockets differ from single stage rockets? Explain.

Answer 1

a) If a black hole passes through a cloud of interstellar matter, it will draw matter inward in a process known as accretion. In this case, the black hole can tear the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate into space which can be detected by X-ray telescopes. Black holes have a significant influence on the neighborhoods around them - emitting powerful gamma ray bursts which can also be detected.
b) A multistage rocket is a rocket that uses two or more stages, each of which contains its own engines and propellant. The main reason for multi-stage rockets and boosters is that once the fuel is burnt, the space and structure which contained it and the motors themselves are useless and only add weight to the vehicle which slows down its future acceleration. By dropping the stages which are no longer useful, the rocket lightens itself. The thrust of the future stages is able to provide more acceleration than if the earlier stages were still attached, or than a single, large rocket would be capable of. When a stage drops off, the rest of the rocket is still travelling near to the speed that the whole assembly reached at burn-out time. This means that it needs lesser total fuel to reach a given velocity and/or altitude.
On the contrary, single stage rocket uses a single rocket motor or a cluster of motors to lift its payload to the desired altitude, and most low altitude sounding rockets are of the single stage type