Gravity, air Resistance, Falling

The website (https://van.physics.illinois.edu/qa/listing.php?id=104 )states that: "If you are comparing a big ball and a little ball that are solid and are made of the same material, then air resistance will slow the big one down LESS than the small one because the big ball is so much heavier than the small one." But a fundamental law of physics is that the mass of a ball has no effect on the speed of a falling ball. Have I made a mistake?
- Will (age 16)
Lausanne, Vaud, Switzerland

One way to help sort out such issues is to think of extreme cases. If neither the shape nor the size of the ball matters, think of a ball that's made out of protein stretched out into many fine little rods- in other words, a feather. Does it fall at the same rate as a golf ball? The world gives you the answer, without having to think about school or books.

You've described a basic law of simplified, idealized physics, in which factors like air resistance are neglected. Laws like that are wonderful tools to help grasp basic principles, but ought not be applied without considering a realistic full physical picture. We live on a planet with an atmosphere. It creates frictional forces.

Mike W.

(published on 01/28/2015)