How come in free fall you feel weightless even though gravity is pulling down on you?

When I look at the tyres of a moving vehicle in a movie the look as if they are moving in the opposite direction. How can we explain this phenomenon/effect?

What is the essence of an oil molecule in hydraulic machines?

How would you measure the density of an irregular solid such as a glass stopper?

Distingush between Bernoulli’s effect and Bernoulli’s principle.