Answer:
Light does not need a medium to travel travel through, but since waves must have a medium to vibrate, sound is not created where no air is present.
Explanation:
Gravitational force is given by, 
Where, m and M are the masses of the objects, R is the distance between them and G gravitational constant.
Gravitational force of the star on planet 1, 
Gravitational force of the star on planet 2, 
Ratio, 

Therefore, the gravitational force of the star on the planet 1 is three times that on planet 2.
The answer is the less dense plate slides over the denser plate.
Answer:
protons
atomic number
Note: The same atomic number can be associated with several different values of atomic mass, but an element can have only 1 atomic number,
A green liquid becoming a red liquid