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Suppose a football is racing towards you at some speed $v$. What you are gonna do is try to stop it. There are two scenarios:

1.Suppose you hit it with your closed fist in the direction opposite to the ball's motion. The ball will rebound as a result.

2.You try hitting the ball,but this time with your palm open and facing the ball. This time also the ball will rebound after hitting your palm but to a lesser extent.


My question is why?

Why when you hit it with closed fist, it rebounds better than it does when you hit it with your palm open. It feels like there is more damping to the ball's motion in the second case... Why so?


Is it that because in the first case, the pressure on the ball is more than in the second case, so it compresses more in the first case than in the second. So we can see, more of the kinetic energy of the ball converts to potential energy(and therefore again into kinetic energy) due to compression... in the first case than it does in the second case... Is it the reason for more damping in the second case...?

Or is there any other way of explaining the damping?

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5 Answers 5

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The palm provides cushioning to extend the time of contact making the collision more inelastic. Collision with fist involves directly pointed bones which deform considerably less than soft flesh on our palms thereby lose less energy and rebound farther. Imagine dropping the ball on a hard floor and then on a sofa. You'll notice the difference in rebound height. Also, on more practical terms, while hitting fists are in line with whole of hand so no place for them to bend back, while palms would dangle above hand and may bend a little backwards allowing even greater time of impacts.

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  • $\begingroup$ Thank You.....๐Ÿ˜Š $\endgroup$
    – user266637
    Aug 16, 2020 at 4:20
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Well I am a footballer myself.

I will always prefer to punch a fast moving ball than to try and stop it with open hands.The reason for this lies a little in Biology. I,as a footballer,know that with open palm,my hands may face greater damage.

This can be explained in numerous ways - force , energy , impulse etc.

Impulse

With fists closed , I supply a much greater impulse to the ball than I do with palms. Hence the momentum change is more in the ball and it flies off.

Energy

With fists closed I supply a much greater energy to ball,hence its final kinetic energy is much higher.

Force

I supply a greater force with fists closed and the ball (yeah you guessed it right) flies off.

Biology

Following Newton's third law,if I apply large force on the ball,my hands will suffer greater reaction force.My closed fists can sustain the force.But my open palms cannot and may fracture.


P.S.

1.Note all explanations are complementary.

2.Football injuries may be very serious. Do not try to experiment with the laws of Physics while at field.

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  • $\begingroup$ Thank You.....๐Ÿ˜Š $\endgroup$
    – user266637
    Aug 16, 2020 at 4:20
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This is because the palm behaves like a cushion for the ball while the closed fist which is hard and bony behaves like a hard floor for the ball. The cushioning effect of the palm makes the collision with the ball more inelastic as compared to that of in case of the closed fist. Moreover, the closed fist do not bend during the collision while the open palm does which increases the time of collision and decreases the impact. As a result of which, greater amount of energy of the collision gets dissipated and less amount of energy gets transferred to both the ball as well as the hand in case of collision with the palm. So yes, there is more damping to the ball's motion in the second case than in the first case due to the above reasons.

I hope it helps you.

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    $\begingroup$ Thank You.....๐Ÿ˜Š $\endgroup$
    – user266637
    Aug 16, 2020 at 4:19
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I am posting another answer to evaluate your reasoning. I might have added this to my previous post but I fear that it will make it necessarily long.


Is it that because in the first case, the pressure on the ball is more than in the second case, so it compresses more in the first case than in the second. So we can see, more of the kinetic energy of the ball converts to potential energy(and therefore again into kinetic energy) due to compression... in the first case than it does in the second case... Is it the reason for more damping in the second case...?

Is it that because in the first case, the pressure on the ball is more than in the second case, so it compresses more in the first case than in the second.

See pressure is just Force per unit area. I do not understand why you prefer to call it pressure.Anyways it does not make a difference.

So we can see, more of the kinetic energy of the ball converts to potential energy(and therefore again into kinetic energy) due to compression... in the first case than it does in the second case...

Well if you take the system to be only ball then you are supplying energy to the system by punching. Your argument has a major flaw that you do not take into account this energy.Assuming collision to be elastic - ball regains shape completely. The initial and final Kinetic Energy,in your argument, will be same following momentum conservation and the fact that you did not gain any noticeable velocity while punching the ball.

When you take into account the energy supplied,the Kinetic energy may be different.

Is it the reason for more damping in the second case...?

It does not matter how much Kinetic Energy is converted into Potential energy during the collision,assumed elastic as there will no Potential energy in the ball after collision.This means that the velocity of the ball does not depend upon how much Kinetic Energy is converted into Potential energy during the collision and only on the energy supplied by your action.

is there any other way of explaining the damping?

Tons.

Finally the fact that you can supply more energy to the system by punching is because you fear that you might fracture your hand in attempt of suppling same energy by palm.

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  • $\begingroup$ Thank you.....๐Ÿ˜Š $\endgroup$
    – user266637
    Aug 16, 2020 at 4:19
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When using the palm open, part of the energy of the collision rotates (a little) the hand around the wrist, what doesn't happen with a closed fist.

The palm itself is a complicated array of articulated bones, that will also move (a little) with the impact, draining still more energy.

It is the same principle that running on the sand requires more efffort that running on a hard soil. The sand moves at the impact, draining energy.

Anyway it is better to use feet instead of hands, because it is football!

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  • $\begingroup$ Thank You.....๐Ÿ˜Š $\endgroup$
    – user266637
    Aug 16, 2020 at 4:19

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