# Why do balls spin?

When you throw a ball, why does it spin?
Does it have to do with the air resistance?
Obviously, you can spin it when you throw it, but it seems to spin even if you just "push" it up.
Is it that I'm just not being precise here?
Is this related at all to why it's easier to balance on a bike when you're going faster?

Related Classical Physics News on Phys.org
Drakkith
Staff Emeritus
A ball doesn't have to spin when thrown. Look at a shotput. It doesn't spin much (if at all) while in flight. Usually a ball spins because when you throw it you put spin on it. Unless you shotput it then you are most likely throwing it similar to how you throw a baseball or football. Since your arm and hand are coming down or across, when you let go of the ball, you give it a final push to one direction as it is coming off of your hand/fingers which puts spin on it.

Take the game of Pool for example. If you hit the Q ball dead center, there is no spin that you introduce on the ball. The reason it rolls is because of friction on the table. But if you move your stick and hit it off center, then you can do all kinds of things due to putting spin on it. In general, you are putting spin on a ball when you throw it, even if it is just a little bit.

Usually when you throw a ball upwards without intending to spin it, it spins because of imperfection. You may have removed contact from one side of the ball while the other part just rolls on the fingers for a fraction of a second . Also you may have pressed the ball differently while holding.

Don't forget the http://en.wikipedia.org/wiki/Magnus_effect" [Broken].

Last edited by a moderator:
Magnus is the deviation of ball when spinning initially. It has nothing to do with this.

russ_watters
Mentor
When you throw a ball, your arm is rotating so it is extremely difficult to throw it without spin. Shotputs are so heavy that you don't so much throw them as push them away from you.

Pythagorean
Gold Member
Even if you could launch a ball with zero rotation, any nonuniformity of the air drag will start the ball spinning towards the side with the highest drag contact point...

well... assuming uniform airflow over the ball :P