According to the text, a star must be hotter than about 25,000 k to produce an h ii region. both the hottest white dwarfs and main-sequence o stars have temperatures hotter than 25,000 k. which type of star can ionize more hydrogen? why?

the part of the wave he is travelling on is the crest

mass and volume are the answers you are looking for

answer: the metamorphic rocks are formed when they are exposed to high pressure and high temperatures. this happens when one of the earth's plates comes in contact with the other plate by traveling in opposite directions, straight into each other or slipping under one another. the friction that is caused by this activity results in   melting of rocks and when it cools they are known as metamorphic rocks. this process needs a controlled amount of pressure and temperature otherwise it may turn into magma.

the moving of the box is a result of centripetal force. think of that  as a car or a truck in this case creates a turn, the force of friction performing upon the turned wheels of the car runs centripetal force obligatory for circular motion for the things inside it.