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Pressure and The Curveball

(Click the Play Ball button in the lower left corner if the baseball isn't already on the screen.)

We want to examine how the spin with which a baseball is thrown relates to the pressure of the air flowing around the surface of the ball. Be sure you have chosen Speed at Surface on the Plotter Control Panel, Cleveland on the Baseball Input Panel, and Left Hander on the Airfoil View Panel.

Now we are ready to throw our second pitch. We want to throw a Curveball, so slowly slide the Spin slider all the way to the left or click the word Curveball.

  • WHAT ARE THE CHANGES YOU SEE TAKING PLACE?

  • WHAT HAPPENED TO THE SIDE FORCE AT THE BOTTOM OF THE INPUT PANEL?

    One very important but slight change you may have noted on the Plotter View Panel is that the yellow and green curves are no longer on top of each other. You can now see that the green curve is going higher than the yellow curve.

  • DO YOU THINK THIS MIGHT SOMEHOW BE RELATED TO THE FACT THAT WE NOW HAVE A SIDE FORCE?

    Our graph is showing that the air stream on the right of the ball (green) is moving faster than the air stream on the left of the ball (yellow).

  • CAN YOU THINK OF A REASON WHY THIS HAPPENS?

    The reason has to do with the little bit of friction between the surface of the spinning ball and the air adjacent to it. As the ball rotates in the same direction as the air going by it (the right side), this friction drags the air along a little faster. On the other side (the left) the ball is rotating against the air. The friction causes the air on the yellow side to slow down a little.

    That's fine, but it still doesn't explain the side force. To do that, you need to select the Surface Pressure button on the Plotter Input Panel.

  • WHAT HAPPENS TO THE GREEN AND YELLOW LINES?

    Do you see that the color of the MAXIMUM on the Surface SPEED plot becomes the MINIMUM on the Surface PRESSURE plot? Click back and forth between the Speed at Surface and Surface Pressure buttons and watch what happens.

  • TRY TO STATE THE RELATIONSHIP BETWEEN THE SPEED OF THE AIR AT THE SURFACE OF THE BASEBALL AND THE PRESSURE IT EXERTS.

    At this point you should have derived the relationship that actually causes the ball to curve.

    ---> AS THE AIR MOVES FASTER IT EXERTS LESS PRESSURE. <---

    In the non-rotating Fastball, the air speeds up equally on both sides of the ball (remember the curves were on top of each other?). So the reduced pressure on both sides is the same, and there is no net side force. Therefore, the Fastball goes straight.

    When the ball spins, the slower moving air on one side has more pressure than the faster moving air on the other side. The difference in pressure "PUSHES" the ball into a curved path. This difference in pressure creates the Side Force.

    It's time for the next lesson - The Screwball
    Or go back to Baseball Lessons

    Please send any comments to:
    Curator:     Tom.Benson@grc.nasa.gov
    Responsible Official: Kathy.Zona@grc.nasa.gov