What is the rate of acceleration for a skateboarder going down a hill starting 10 m/s
to 40 m/s in 15 seconds?

Is there any opitions? if you have the periodic table, the noble gases are 8a family, the halogen are the 7a family, and the nonmetal are  carbon, nitrogen, phosphorus, oxygen, sulfur, selenium, fluorine, chlorine, bromine, iodine and astatine.

Planck decided to assume that atoms could only vibrate at certain frequencies that were whole numbers eg 2f, 3f and so on. others used to think that atoms vibrate continuously