What happens when you shake a container of rice that has balls in it?
Will the balls go to the bottom? (gravity) or to the top? or just stay where they are? (inertia)
Shake the container to find out.
Here is the answer:
In any mix of solids there are spaces in-between the pieces. When your mix is shaken, the grains rearrange themselves.
Gravity forces the grains downward. The balls also need a space to move into.
As you shake the jar, each rice grain finds a space. But there is never a space big enough for the balls.
As the balls move up, rice grains settle underneath. They stop the balls from moving back down. With each shake, the balls move up, but not down.
After a few seconds of shaking the balls all sit on top of the rice.
This reminds us of our cereals. The big pieces of cereal are always on the top of the box while the crumbs are always at the bottom. Can you tell why this is? How can we get the big pieces on the bottom with the crumbs on top?