Which substance is a combination of different atoms?

In science and physics what is the standard unit of measure for speed?

Consider a system to be two train cars traveling toward each other. what is the total momentum of the system before the train cars collide?   kg • what must the total momentum of the system be after the train cars collide?   kg •  

Holes drilled several kilometers into earth’s crust provide direct evidence about earth’s interior in the form of