WHY? The speed of the ball is changing throughout its flight. Air resistance and its interaction with the court both reduce its speed considerably. The radar gun only measures its instantaneous speed right at the start, whereas the speeds that you calculated averaged in a lot of slower travel as well. By the time it reaches the far end of the court, a serve may have lost as much as half of its speed. ASSESSMENT: Can the student communicate what they observed?

Student is able to communicate that the ball is losing speed due to air resistance, as is evidenced by the fact that the average speed for the first flight is lower than the radar gun measurement. The ball also loses speed during the bounce, as is evidenced by the average speed of the second flight, which is much lower than that of the first. Student is able to communicate that ball is losing speed throughout its flight, and that the average speed of the second flight is lower than that of the first. Student is able to communicate the average speed of the ball is lower than the radar gun measurement, because the ball has slowed down during its flight. Student is able to communicate that the ball has slowed down during its flight.

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Last modified: Tue Nov 11 10:22:50 PST 1997

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