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Old 08-24-2012, 12:14 PM   #66
Join Date: Aug 2007
Posts: 957

Originally Posted by sureshs View Post
The force has already driven the acceleration of the racket, that is why it has reached that velocity! Moreover, the force that acts during collision is huge and when multiplied by the small time is called the impulse. The force is called the impulsive force. See the example of the magnitude of the force below. toly is saying that the force is negligible!!!!

The impact between bat and ball is an extremely violent one, in which the bat imparts a huge force on the ball thereby causing it to change directions and gain speed. Consider a baseball weighing 5.125oz (mass = 0.145kg) which approaches the bat at a speed of 90mph (40.2m/s). After the collision with the bat, with a contact time of 0.7milliseconds (0.0007s)[1,2] the bat has a speed of 110mph (49.1m/s) in the opposite direction. Using Newton's second law we can estimate the average force acting on the ball during the hit:

Plugging in the numbers we find the average force to be Favg=18,436 N, which is equivalent to 4124 lbs of force.
Suresh, the force we need to worry about is the external one being applied by the player wielding the racquet. That is a component that could skew the otherwise perfect equations of momentum and energy conservation, should it be significant enough.

Of course, the racquet and ball exert huge forces on each other. The equation for conservation of momentum will apply if that is the only force we need to worry about. The equation for energy conservation will also apply if the collision is perfectly elastic. However, an external force like the player can skew things, but in this case, as pointed out earlier, its contribution in terms of energy during the collision period can be ignored for the purposes of getting an approximate result.
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