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A tennis player is able to exert an average force of 28N when hitting a ball.
Calculate the impulse if the racket is in contact with the ball for 0.1 seconds:
Then I did: 28 x 0.1 and got 2.8 kg m/s.
The follow up question was: A 70 g tennis ball comes toward the player at 30 m/s and it hit back in the opposite direction. Show that if the contact time between the racket and ball is 0.1s, the ball leaves with a velocity of 10m/s.
So the teacher did: Change in Momentum = mv - mu
2.8 = 0.07 x 30 - 0.07 x (-10)
Ok, I tried to do it with my thinking and I got this:
The initial mv is: 30 x 0.07 = 2.1 kg m/s
The final mv is: -10 x 0.07 = -0.7
Which, equals to -2.8, which, might have been because he did initial - final momentum? Or did I do something wrong?
But anyway, the greatest problem is the fact I don't know how he knew beforehand that the change in momentum would be 2.8! I know we had to, because it asked us to SHOW. My difficulty is due to the fact that from the first question, I thought that impulse was generated when the ball was hit by the tennis player from a standstill, since a ball with a different initial velocity would have started with a lot more momentum and consequently, the change would've been much greater by the end.
I am confused, any help appreciated, thanks.
Calculate the impulse if the racket is in contact with the ball for 0.1 seconds:
Then I did: 28 x 0.1 and got 2.8 kg m/s.
The follow up question was: A 70 g tennis ball comes toward the player at 30 m/s and it hit back in the opposite direction. Show that if the contact time between the racket and ball is 0.1s, the ball leaves with a velocity of 10m/s.
So the teacher did: Change in Momentum = mv - mu
2.8 = 0.07 x 30 - 0.07 x (-10)
Ok, I tried to do it with my thinking and I got this:
The initial mv is: 30 x 0.07 = 2.1 kg m/s
The final mv is: -10 x 0.07 = -0.7
Which, equals to -2.8, which, might have been because he did initial - final momentum? Or did I do something wrong?
But anyway, the greatest problem is the fact I don't know how he knew beforehand that the change in momentum would be 2.8! I know we had to, because it asked us to SHOW. My difficulty is due to the fact that from the first question, I thought that impulse was generated when the ball was hit by the tennis player from a standstill, since a ball with a different initial velocity would have started with a lot more momentum and consequently, the change would've been much greater by the end.
I am confused, any help appreciated, thanks.
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