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I am trying to understand the physics behind why and how a ball stretches when bouncing.

Two questions I have regarding this:

  1. Does the amount of stretch depend on the velocity vector? If so would a ball in a vacuum stretch when moving with a constant velocity?

  2. Most common depictions of a bouncing ball show the ball stretch in the direction of movement (ie the velocity vector). Is this accurate? My intuition tells me the ball should stretch in the direction of force / acceleration vector. So if a ball bounces while moving forward horizontally, it should stretch in the vertical direction only and not in the direction of the velocity vector?

enter image description here

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I have never seen a picture like this, why it should stretch just before hitting? The ball does not "know" the Wall or bottom will be reach in the next moment. The ball is a littel deformed by the heavy contact to the bottom but not so uniformely and will be compressed by hitting. Maybe the picture shall just indicate the speed of the ball so one may see it as if it is prolonged, So the axes of the ball are something like the velocity vector.

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  • $\begingroup$ I think the general idea is that the faster the ball goes, the more it will stretch. It doesn't deform only as a result of contact with a wall. A real life example is tennis ball. A slowed down footage shows that when thrown up, the tennis ball is stretched until it reaches the top. $\endgroup$
    – Mushegh
    Oct 9, 2022 at 19:56
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    $\begingroup$ There is no reason for it to stretch! So I doubt that you really have a slow moving picture showing stretching. $\endgroup$
    – trula
    Oct 9, 2022 at 20:09
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A ball does not stretch in the manner shown in the animation image, nor does a tennis ball stretch as you suggest in your comment to trula's answer.

The animation image: the stretch in the picture is just artistic licence - it is used by an animator because it unconsciously leads the viewer of the animation to anticipate the collision of the ball with the floor. In reality, teh ball does not deform at all before it hits the ground. In practice, the ball will only deform:

  1. while it is incontact with the floor, as a result of elastic compression; and
  2. possibly immediately after it has left teh floor, as a result of internal 'wobbling'.

But unless the ball is very soft or moving very fast, there is unlikely to be any discernable deformation even during the collision.

The tennis ball comment: This is just an optical illusion resulting from how photography works. During each frame, the camera shutter is open long enough for the balls position to move a little. As a result the ball is 'smeared' in the image and appears elongated. In fact, if you know the shutter-open time of your camera (or the electronic collection period for a shutterless camera), then from the length of the 'smear' you can tell exactly how fast an object is moving. But in reality there is no physical elongation.

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