Why february contains 28 or 29 days?

Photoelectrons with a maximum speed of 6.50 x 107 m/s are ejected from a surface in the presence of light with a frequency of 6.75 x 1014hz. if the mass of an electron is 9.10 x 10-31 kg, calculate in joules the maximum kinetic energy of a single electron. 3.84 x 10-15 j 1.92 x 10-15 j 5.92 x 10-23 j 3.07 x 10-16 j

Considering only the earth's rotation, determine how much later the asteroid would have had to arrive to put the explosion above helsinki at longitude 25˚ e? this would have obliterated the city.

Consider an ideal gas at 27.0 degrees celsius and 1.00 atmosphere pressure. imagine the molecules to be uniformly spaced, with each molecule at the center of a small cube. what is the length l of an edge of each small cube if adjacent cubes touch but don't overlap?