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Old Mar 18th 2013, 08:28 AM   #1
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Kinetic and potential energy

I am having some problem understanding this question,

A batsman strikes a ball of 500 gm, from 1m above the ground,upwards to a height of 10m and the wicket keeper catches the ball at almost the same point and the same level from the ground. Find:

a) Kinetic energy with which the ball strikes the gloves of the keeper.
b) Velocity of ball immediately after striking the bat.
c) Potential energy of the ball 5m above the ground.
d) Net work done against gravity.

What i particularly don't understand is "the wicket keeper catches the ball at almost the same point and the same level from the ground."

Please help i am really confused.
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Old Mar 18th 2013, 08:51 AM   #2
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Originally Posted by noddy View Post
I am having some problem understanding this question,

A batsman strikes a ball of 500 gm, from 1m above the ground,upwards to a height of 10m and the wicket keeper catches the ball at almost the same point and the same level from the ground. Find:

a) Kinetic energy with which the ball strikes the gloves of the keeper.
b) Velocity of ball immediately after striking the bat.
c) Potential energy of the ball 5m above the ground.
d) Net work done against gravity.

What i particularly don't understand is "the wicket keeper catches the ball at almost the same point and the same level from the ground."

Please help i am really confused.
The ball went straight up .. or at least close enough to straight up that the horizontal component of motion can be neglected. The plot of height as a function of time is a parabola, and by symmetry the speed coming back down will be the same as the initial upward speed (final velocity is the negative of the initial). Since the initial and final vertical positions are the same, you can consider the elevation of 1 m to be the zero of the potential energy.
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Old Mar 18th 2013, 09:18 AM   #3
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Ok, so does that mean, that the wicket keeper caught the ball when it came down to a distance of 1m from the ground ?
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Old Mar 18th 2013, 11:21 AM   #4
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Originally Posted by noddy View Post
Ok, so does that mean, that the wicket keeper caught the ball when it came down to a distance of 1m from the ground ?
Yes. You can assume the point of the catch is the same as the point at which the ball left the bat.
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Old Mar 18th 2013, 05:53 PM   #5
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Thank you ! got it now .
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Old Jan 3rd 2014, 12:21 PM   #6
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Originally Posted by noddy View Post
Thank you ! got it now .

There's only twe things that anybody needs about potential energy. They are

(1) The potential energy function U is defined such that a particle in such a potential experiences a force given by F = -grad U

(2) If the potential function is not a function of time then the sum of kinetic energy and potential energy is a constant of motion.
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