Naked Science Forum

General Science => General Science => Topic started by: Chemistry4me on 20/01/2009 07:52:27

Title: Why does a basketball spin in the opposite direction after it hits the ground?
Post by: Chemistry4me on 20/01/2009 07:52:27

If I spin a basketball clockwise and let it drop onto the ground, it bounces back up spinning anticlockwise  [:o] Why is that? [???] [???]

Title: Why does a basketball spin in the opposite direction after it hits the ground?
Post by: Karsten on 20/01/2009 22:08:08

I  think it is a result of the ball's surface sticking to the ground for a moment, being stretched sideways (along its circumference), and delivering a reaction opposite to the previous action. Balls that don't stretch well or are not sticky enough should not exhibit this behavior.

Try it: Bounce it on ice. Well, you will have to wait a few months and I don't have a basketball.

If you take a rubber ball give it a spin and throw it forward it will jump short (=spin against forward motion) and long (=spin with forward motion) in alternating fashion.

Title: Why does a basketball spin in the opposite direction after it hits the ground?
Post by: lyner on 20/01/2009 22:13:24

I wrote this whilst the above post was arriving. This is my two pennyworth.

The 'skin' of the ball is elastic. When it hits the floor, the patch in contact with the floor stops moving (momentarily stuck) but the rest of the skin is still rotating, building up tension one side and compression the other. These forces act to slow the rotation down; elastic energy is stored. They reach a maximum when the ball stops rotating. Before the ball leaves the ground, the forces on the surface star to rotate the ball in the opposite direction; the elastic energy is returned.
If the ball is in contact long enough, the ball will rotate at an equal rate as when it arrived, but in the opposite direction.

Title: Why does a basketball spin in the opposite direction after it hits the ground?
Post by: Karsten on 20/01/2009 22:20:16

I am honored to have guessed what you know and have phrased so much more eloquently than me.

Title: Why does a basketball spin in the opposite direction after it hits the ground?
Post by: Chemistry4me on 20/01/2009 23:18:57

Cheers Karsten and sophiecentaur, that explains it! Very well put.