Why Does a Ball Spin When Thrown?

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Discussion Overview

The discussion explores the reasons why a ball spins when thrown, examining factors such as air resistance, throwing technique, and the influence of imperfections in the throw. It touches on both theoretical and practical aspects of ball dynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that a ball typically spins when thrown due to the way it is released, with the arm and hand motion imparting spin.
  • Others argue that a ball does not necessarily have to spin, citing examples like shotputs which are thrown with minimal spin.
  • One participant notes that imperfections in the throw, such as uneven contact with the fingers, can lead to unintended spin.
  • The Magnus effect is mentioned as a relevant phenomenon, although its direct connection to the initial spin of the ball is debated.
  • Another viewpoint emphasizes that the weight of the shotput affects how it is thrown, suggesting that heavier objects are pushed rather than thrown with spin.
  • It is proposed that even if a ball could be launched without rotation, variations in air drag could induce spin during flight.

Areas of Agreement / Disagreement

Participants express differing views on whether a ball must spin when thrown, with some asserting that it typically does due to the throwing motion, while others maintain that it can be thrown without spin. The discussion remains unresolved regarding the extent to which air resistance and imperfections contribute to spin.

Contextual Notes

There are limitations in the discussion regarding assumptions about the uniformity of airflow and the specific mechanics of different throwing techniques. The relationship between air resistance and spin is not fully explored.

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

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