He noticed that a ball picked up speed if rolled down a ramp, and it lost speed when it rolled up a ramp. Any moving body, even a moving body rolling across a seemingly flat plane, will roll forever, an effect Galileo called inertia. Friction is the force that interferes with this motion. In this case, friction caused by the slightly uneven surface of the tabletop creates a drag on the rolling object, which will eventually come to a stop. A batted baseball traveling through the air in a high altitude place like Mexico City, will travel further than a ball at sea level.
Air resistance caused by friction is also a friend to falling cats.
Well, cats that live in high rise apartment buildings have been known to loose their balance, and fall out of a window. The mortality rate is twice as high for cats that fall six stories or less. Well, the cat knows that increasing its mass leads to greater friction or air resistance. So, falling cats right themselves and spread their legs to form a sort of kitty parachute. The uprushing air has a bigger area to push against.