# Rotational ball

1. Feb 24, 2008

### Skhandelwal

When a ball spins....does it always have one point which is spinning faster? Meaning is it possible for a spinning ball to have an axis at center? If one part is spinning faster....then is the slowing spinning part directly opposite(180 degrees) to that point?
When the fastest point is spinning at 30rpm, how can determine the speed of slowest point?

2. Feb 25, 2008

### tiny-tim

axis of rotation

I don't really understand this question.

A spinning sphere always has an axis of rotation. The two points where that axis meets the surface of the sphere will be stationary (so they will be moving the slowest)(and so, of course, wil be the points on the axis inside the sphere). The "equator" half-way between them will be moving fastest.

But the rpm is the same for every point on the sphere - if the equator is spinning at 30rpm, then so is the remainder of the sphere (including the points inside).

3. Feb 25, 2008

### Skhandelwal

really...but I was told that the ball curves is b/c parts of is spinning faster than the other....(I am not a talking about a tilted ball) The difference in air pressure creates the curvature....

4. Feb 25, 2008

### tiny-tim

curve of a spinning baseball

Ah! You're talking about the spin that a pitcher puts on a baseball, that makes it curve in flight!

Yes, it's like the flow over an aeroplane wing - the flow over the top of the wing is faster than the flow under it, and that creates a difference in air pressure, which lifts the wing (I think I've got that the right way round).

Similarly, if the baseball is spinning clockwise, then the points on the left will be going slightly faster than the centre of the ball, and the points on the right will be going slightly slower, so the air is going faster on the left than on the right, and so the ball will curve to the … left (I think).

If by "spinning" you mean "moving", then yes - velocity is "additive", so if the velocity of the centre of the ball is V, and the velocity of a particular point on the surface is V + W, then the velocity of the opposite point must be V - W.

In particular, the fastest point is always directly (180 degrees) opposite to the slowest point.

rpm isn't a speed. You need fps.

5. Feb 25, 2008

### ZapperZ

Staff Emeritus
You sometime need to really sit down first and think about what you are saying here. It makes no sense to say "parts of the ball is spinning faster than the other". How can that occur for a rigid body in this simple situation?

What is relevant here is the air speed that is flowing over the different parts of the surface as the ball spins and moves through the air. This is where there is a difference in different parts of the ball.

Zz.

6. Feb 25, 2008

### rcgldr

That's Magnus effect. One explanation is the thin layer of air attached to the spinning ball, creates a direct differential in air speed relative to the ball, accelerating some air perpendicular to the path of the ball which causes the ball to accelerate in the opposing direction, which causes the curve. The first link below mentions this and includes a diagram of "top spin", the net effects are air is accelerated upwards, so the ball is accelerated downwards. The other explanation is that the layer of air attached to the ball is too thin to be directly responsible for "lift", but causes the air flow around the ball to detach on the forward moving side sooner than the backwards moving side of a ball. The second link mentions this and includes a diagram of "back spin", air is accelerated downwards and the ball is accelerated upwards.

Airspeed diffferential explanation:
http://en.wikipedia.org/wiki/Magnus_effect

Seperation of air stream explanation:
http://www.geocities.com/k_achutarao/MAGNUS/magnus.html

Using the air itself as a frame of reference, the horizontal flow over the top of the wing is slower than the horizontal flow below the wing. However, with the air as a frame of reference, the dominant component of air flow is downwards (corresponding to lift), with a smaller component of air flow forwards (corresponding to drag).

Unlike the spinning ball, where seperation of air streams seems to be the more likely explanation of spinning ball "lift", wings simply have an effective angle of attack which when combined with air speed, accelerates the air downwards. For a "normal" wing design, most of this downwards acceleration of air occurs above the wing, partly because the low pressure area above the wing "steals" some of the air flow that would go under the wing. However there are wings with flat tops, curved bottoms that fly just fine, as in this case of some pre-shuttle prototypes for space re-entry vehicles called flying bodies:

M2-F2 with F104 chase plane.jpg

From this web site:

http://www.dfrc.nasa.gov/Gallery/Photo/index.html

The M2-F2 was a glider prototype for the M2-F3 which was powered by a rocket engine and achieved speeds of mach 1.6, so in spite of it's appearance, it wasn't a high drag wing. The main change to the M2-F3 was to add a 3rd vertical stabilizer at the back.

Last edited: Feb 25, 2008