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Force on the Wiffleball

The first thing to remeber is that there are 3 forces that act on a wiffleball in flight.

1) Gravity
2) Uneven pressure created by the holes
3) Wind friction

1) Gravity:

If you don't know what it is stop now. It's values is (32 ft/s^2), downward of course.

2) Uneven pressure created by the holes:

The main force the Wiffleball players use is the lift created by the hole in the ball. This force work just like the wing of an airplane.

The air on the top of the wing (curved side) must travel farther in the same amount of time as the air on the bottom. Hence, it will travel faster. This creates a low pressure zone on the top and high pressure on the bottom. And this will create lift.

Now that we understand that lets look at a wiffleball.

For the ball to have lift, it is thrown with the holes on one side and the solid side on the other. When this happens some of the air flows through the holes, which makes a shortcut for the air. The air on the hole side will therefor travel slower. Just like the plane, a high pressure zone builds up on the side on the side with the holes and a low pressure zone is created on the solid side.

This force acts on the ball perpendicular to the "equator" against the side with the hole.

3) Wind friction:

If you are looking at the box that a wiffleball comes in, the above is wrong. They have the ball curving in the opposite direction.

The reason for this is wind friction. While the above is correct, it is only correct for higher velocities. For lower velocities (lob) the wind friction out weighs and overcomes the uneven pressures.
The reason that wind friction has any uneven forces on the ball is also because of the holes. The edges of the holes (ie. thickness of the ball) and possibly some air turbulence inside the ball, slows that half of the more than the smoother more airodynimic side. This will tend to turn the ball towards the side with the holes. This acts just as car driving with one side of its wheels on the road and one side in sand.

Quick example:

A ball thrown with the equator staight up and down, and the holes on the right, reference from the pitcher.

3 forces...
Gravity, it acts down.
Uneven pressure created by the holes, this will force the ball to curve to the left.
Wind friction, this will force the ball to curve to the right.

Which way the ball will curve, depends on the velocity on the ball and any scuffings on the ball. At higher velocities the ball will curve to the left.