Understanding the Football Rotation Axis: A Gr. 10 Student's Explanation

In summary, the movement of a football when spun by hand is due to the forces of gravity and the reaction force against the floor, causing a moment to be applied to the ball. The ball acts as a gyroscope, with the rotation axis being turned in an unexpected direction. Once the ball is upright and rotating around the axis with the smallest moment of inertia, it is in a stable state. However, this is not the case for rotating machinery, which requires more than two blades to prevent instability. The stability of rotational axes is also responsible for the football standing up when spinning, but this is due to friction. Without friction, the opposite effect would occur.
  • #1
bassbreaker
1
0
i only joined this site to post this thing... now today at school (im in gr 10 by the way) a gym teacher came up to me with a football (american) in his hand and said "bassbreaker (not really my name but we'll leave it at that) You're a smart kid, tell me how this happens." and he rotated the football on its side. Then after a couple of seconds it moved up and it was rotating on its long axis. I was like WTF?! then i said to him some bull **** excuse like, "well because of the axis the Earth spins on, taking gravity into the picture, the circumphrence of the atmospheric..." that's when he was like, ok dere, lies. But w/e i don't know how it works its probably like something to do with balance or sumthing... w/e but I've been looking all over the net n it didnt say it so please someone answer
 
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  • #2
A simple equation-free explanation:

The forces on the football are gravity (acting at its CG) and the reaction force against the floor.

If the football is not rotating in a perfectly symmetrical way, those forces are not lined up exactly so they apply a moment to the football. If it was spun by hand, the initial motion would not be perfectly symmetrical.

The football is effectively a gyroscope, and applying a moment turns the rotation axis in an "unexpected" direction just as with a more conventional looking gyroscope.

Once the ball is upright and rotating about the axis with the smallest moment of inertia, it is in a stable state similar to a spinning top, until it slows down (because of friction and air resistance), starts to wobble, and falls down again.

Incidentally this is a real issue when designing rotating machinery. There is a good reason why large wind turbines etc have more than two blades - if they had only two blades, they would tend to be unstable and try do the same thing as your football! With more than two blades, this can't happen.
 
  • #3
AlephZero said:
Once the ball is upright and rotating about the axis with the smallest moment of inertia, it is in a stable state
This reason is wrong.

The more stable state is the one with minimum kinetic energy (for a given quantity of angular momentum), because that state will be approached as a spinning object internally dissipates kinetic energy as heat. Since angular momentum is proportional to angular velocity whereas kinetic energy is proportional to the square of that (and both are directly proportional to moment of inertia), kinetic energy is minimised by rotating around the axes with the greatest moment of inertia (which is how the ball begins).

[See, for example, http://einstein.stanford.edu/highlights/hl_polhode_story.html" .]

The football standing up is a completely different effect, which is complex but "well known" (http://scholar.google.com/scholar?q=spinning egg") for the case of a spinning egg, and mainly has to do with friction. Without friction, as explained above, the stability of rotational axes produces the exactly opposite result.
 
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FAQ: Understanding the Football Rotation Axis: A Gr. 10 Student's Explanation

What is the football rotation axis?

The football rotation axis refers to the imaginary line that runs through the center of the football and is perpendicular to its surface. This is the axis around which the football spins when it is thrown or kicked.

How does the rotation axis affect the flight of the football?

The rotation axis plays a crucial role in determining the trajectory and stability of the football's flight. As the football spins around this axis, it creates a gyroscopic effect which helps to stabilize its flight and maintain its orientation.

What factors influence the rotation axis of a football?

The rotation axis of a football can be influenced by several factors, including the angle at which it is thrown or kicked, the velocity of the throw or kick, and the rotation of the ball itself. Additionally, external factors such as wind and air density can also affect the rotation axis.

Why is understanding the rotation axis important in football?

Understanding the rotation axis is crucial in football because it allows players to control the trajectory and accuracy of their throws and kicks. By understanding how the rotation axis affects the ball's flight, players can make adjustments to their technique and improve their performance on the field.

Can the rotation axis be manipulated in football?

Yes, the rotation axis of a football can be manipulated by changing the angle and velocity of the throw or kick, as well as by using techniques such as spin or knuckleballs. However, these manipulations require a high level of skill and precision and may not always produce the desired result.

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