Fine grains of beach sand are assumed to be
spheres of radius 38.9 µm. these grains are
made of silicon dioxide which has a density of
2600 kg/m3
.
what is the mass of each grain of sand?
answer in units of kg.

what experimental evidence led scientists to change from the previous model to this one?

a few of the positive particles aimed at a gold foil seemed to bounce back.

the colors of light emitted from heated atoms had very specific energies.

experiments with water vapor showed that elements combine in specific proportions.

cathode rays were bent in the same way whenever a magnet was brought near them.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.