Film loop: "Dynamics of a billiard ball", 4:00 min.

Film Loop: Dynamics of a Billiard Ball Length(min.):3:30 Color: No Sound: No The event pictured in this film is one which you have probably seen many times - the striking of a ball, in this case a billiard ball, by a second ball. The camera is used to "slow down" time so that the details in this event will be more evident. The ability of the camera to alter space and time is important in both science and art. The slow-motion scenes were shot at 3,000 frames per second. The "world" of your physics course often simplifies what is actually observed. Thus, in your textbook, much of the discussion of the mechanics of bodies assumes that the objects are point objects with no size. But clearly these massive billiard balls have size, as do all the things you encounter. For a point particle we can speak in a simple, meaningful way of its position and velocity. But the particles photographed here are billiard balls and not points. What information might be needed to describe their positions and velocities? Looking at the film may suggest possibilities for describing the motion of such objects. What motions can you see beside the linear forward motion? Watch each ball carefully, just before and just after the collision, watching not only the overall motion of the ball, but also the "internal" motions. Can any of these motions be appropriately described by the word "spin"? Can you distinguish the cases where the ball is rolling along the table, so that there is no slippage between the ball and the table, from the situations where the ball is skidding along the table without rolling? Does the first ball move immediately after the collision? Even this simple phenomenon is a good bit more complex than you might have expected. Can you write a careful verbal description of the event? How might you go about giving a more careful mathematical description? Using the slow-motion sequence, make a partial momentum analysis of this collision. Measure the velocity of the cue ball before impact, and the velocity of both balls after impact. Remember that there is friction between the ball and the table, so velocity is not constant. The balls have the same mass, so conservation of momentum predicts that Velocity of cue ball before collision = sum of velocities of the ball just after collision. How closely do the results of your measurements agree with this principle? What reasons, considering the complexity of the phenomenon, might account for any disagrement? What motions are you neglecting in your analysis?
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