The Wind Tunnel (Page 4)

Look at two videos of the same ball, at different speeds. You can see the flow passing over the ball at a higher velocity. Notice that the lines are closer together in the faster flow.

Compare the wakes of the videos of topspin, no spin and backspin for the tennis ball or bowling ball with glass beads. Do you see anything different? The wake of the ball with no spin is symmetric. The wake goes straight down the middle behind the ball. Now look at wake of the ball with topspin. The wake is pointed upwards. What about the backspin? The wake is pointed downwards. The fact that the wake is pointed downwards signifies that the ball is actually being forced up. For topspin the opposite is true - the wake pointing up signifies that the ball is being forced down.

You know from playing tennis that this is true - topspin drops the ball faster than backspin. But the direction of the wakes might be a little puzzling at first. Keep Newton's Third Law in mind, "For every action there is an equal and opposite reaction". This applies to the direction of the flight of the ball and the direction of the wake. For an aerodynamicist, that wake deflection tells us that the flight path is in the opposite direction.