What do scientists base their classification of rocks on

What is hidden during a lunar eclipse? isn't the moon hidden?

A2 kg ball travellng to the right with a speed of 4 m/s collidees with a 5 kg ball traveling to the left with a speed of 3 m/s. take right to be the positive direction. what is the total momentum of the two balls before they collide? what is the total momentum of the two balls after they collide?

What are similarities and differences of reflection, refraction, diffraction and absorption?