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This new video from the slow-mo guys captures the explosion of an apple when it is rotated really fast (roughly $109$ rotations per second).

But can someone explain me why did it explode? Can we explain this on the basis of centrifugal force (from the apple's frame)? If yes, what would be the cause from an inertial frame?

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    $\begingroup$ By editing the question to ask about why the apple is cut into clear halves it looks like you invalidated any answers that only answered why the Apple exploded. $\endgroup$ Nov 30 '20 at 21:20
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    $\begingroup$ Greetings! I've rolled back your question to the revision which is consistent with the early answers. Please try to make future edits in a way that doesn't invalidate answers you've already gotten (see e.g. this recent discussion). If the question you wanted to ask turns out to be really different from the question that you actually asked, you can just ... ask the question you meant to from the "new question" page, and link this one for context. Cheers! $\endgroup$
    – rob
    Dec 1 '20 at 3:08
  • $\begingroup$ I think the better question is why would it stay in tact under sufficiently large centrifugal force? $\endgroup$
    – Myridium
    Dec 1 '20 at 6:57
  • $\begingroup$ Hi Ankit. The edit v9 does not seem to improve the question. The centrifugal force plays a central role. $\endgroup$
    – Qmechanic
    Dec 24 '20 at 12:00
  • $\begingroup$ @Qmechanic but isn't it important only in accelerating frame ? $\endgroup$
    – Ankit
    Dec 24 '20 at 12:56
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Symmetry

First, the apple appears to break into 3 parts, not two. Second, the break is obviously not as "clean as a knife" if you slow the video down and watch it carefully. However, the break is much cleaner than if you tried to tear the apple into chunks with your bare hands (unless you happen to be Edward Scissorhands, of course). Apples in general are not designed for high tension, as there is no biological motivation for such a property. On the contrary, apples are basically designed to be consumed by animals, to help propagate the seeds. That is why they have relatively soft flesh on the outside, unlike most nuts.

The other answers have explained why the apple falls apart, so I will try to give an idea for why it breaks into three pieces the way it does. If you look carefully at an intact apple, you will notice that it has a rough radial symmetry about the axis going through the stem. If you study the bottom, in particular, you will notice that quite often, there is a 5-fold symmetry. On top of that, the average apple contains 5-8 seeds. Perhaps you are noticing a pattern here. Biology tends to be very economical (because that is rewarded, and extravagance is penalized with extinction), so many "body plans" look like: "make this widget, and duplicate it N times". For an apple, that appears to be something like: "make this ear-shaped slice and put a seed near the middle, and do that five times in a circle". Of course, it doesn't make the apple fully formed in that shape, but rather starts out growing around the seeds themselves.

When the apple finally breaks, we are then left with the question: "Why does it break where it does?" And as with all mechanical systems, the answer is generally: "That's where the weakest points were." So where are the "weak points" on an apple? Going back to the bottom of the apple, we infer the 5-fold symmetry because of the "bumps" on the bottom, which can range from quite prominent to barely noticeable, if at all. You can imagine that the larger mass/internal surface area near the "peak" of one such bump has higher tensile strength (ability to resist being pulled apart), and that there is a correspondingly lower tensile strength in the "valleys" between the bumps. I would predict that the valleys are thus where the apple is most likely to break. And this also explains why it breaks into 3 pieces, and why the pieces are not the same size. Breaking roughly in half would be the simplest way to release tension force in the apple, but the symmetry makes a clean half-break less likely. Instead, it appears that the apple breaks into more like 2:2:1 fragments. It's not precise, because there isn't a clean division between the "lobes" of the apple.

Given these observations, I would expect that were this experiment to be replicated on an orange, that the orange might be more likely to split cleanly in half, given that oranges typically have 10 sections. In addition, the lobes of an orange are quite distinct, resulting in fairly inhomogeneous internal forces upon rotation. On the other hand, I would not be surprised if large-scale (but subtle to the eye) variations in the shape of the orange caused it to split asymmetrically like the apple, but still along the 10-fold symmetry lines.

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  • $\begingroup$ Interesting speculation, but I would like proof this is really happening. $\endgroup$
    – Winston
    Nov 30 '20 at 19:18
  • $\begingroup$ Actually in the 2nd attempt with the better camera, the apple splits exactly in halves, with one half then splitting further. youtu.be/nedusgCUZC4?t=250 $\endgroup$ Nov 30 '20 at 23:54
  • $\begingroup$ @CrisLuengo I didn't watch the video that far, but it is impressive how cleanly it splits into two. This is clearly a different apple, and when he draws the lines on it, we see that it has a high radial symmetry. You don't see much of the typical "bottom bumps". However, in a few frames, the lighting reflections appear to imply a 6-fold symmetry to me. It's very subtle, but it looks like faint 120 degree dents in the bottom. Therefore, it is less surprising that this apple broke cleanly in half. The second fragment was probably affected by the air source when it broke. $\endgroup$ Dec 1 '20 at 1:53
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Let's think of a simplified system: two point masses connected by a massless string. As the masses circle around the center of the string, the string provides the centripetal force required to make the masses turn in a circle. We can find the tension by using Newton's second law. Ignoring gravity, we recognize that the tension is the only force acting on each mass, and the acceleration is centripetal acceleration.

$$ F_\mathrm{net} = T = m a_c $$

The tension depends on the mass on each end $m$, the length of the string $\ell$, and the angular frequency $\omega$. The centripetal acceleration to turn is: $$a_c = \frac{v^2}{r} = \frac{(\omega r)^2}{r} = \omega^2 r = \omega^2 \frac{\ell}{2}. $$

So the tension in the string is: $$ T = \frac{m \omega^2 \ell}{2}. $$

A real string will eventually break if the tension gets too great. If it spins too fast (big $\omega$), then the string breaks.

From the point of view of an inertial frame the tension is required to change the direction of the masses velocity. The acceleration turns the masses in a circle.

Using a co-rotating frame, the the tension is balanced by the centrifugal pseudo-force.

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  • $\begingroup$ So when they start rotating the tension increased (from inertial frame) and thus it breaked ? $\endgroup$
    – Ankit
    Nov 29 '20 at 17:51
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    $\begingroup$ The physical reason it breaks (i.e. the tension) is the same in any coordinate frame. The apple doesn't magically "know" what coordinate frame you are using! $\endgroup$
    – alephzero
    Nov 30 '20 at 2:50
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    $\begingroup$ @Ankit It's gotta break somewhere. You can see the crack begins on the surface, then expands perpendicular to the surface. Tug on opposite sides of a piece of silly putty and it'll break in much the same way - the tear will be perpendicular to the surface. So the question is why doesn't the tear go diagonal or curve? $\endgroup$ Nov 30 '20 at 6:28
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    $\begingroup$ @Ankit, It's a question of least-resistance. Basically, since the apple's "meat" is fibrous, and those fibers run from top to bottom perpendicular to each other, when the entire apple is under a relative uniform stress, it's very likely that the break will happen between the fibers, forming a relatively straight line $\endgroup$
    – Neowizard
    Nov 30 '20 at 8:32
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    $\begingroup$ @Neowizard You mean parallel to each other? $\endgroup$
    – user253751
    Nov 30 '20 at 13:29
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Breaking into parts is due to the structure of the apple, which I can't elaborate on because I'm not an apple biologist, but it might be illuminating to think of what would happen in a simpler case. If the object spinning is elastically deformable (think of a bouncy ball or a stress ball, if you know what that is) then, as explained in the other answers, it will stretch out in the plane perpendicular to the axis of rotation because you can effectively replace the string described by Paul T. with a spring. As the tension force increases, the spring stretches, and the non-rigid ball deforms in a similar way:

enter image description here

Eventually as the RPM increases, these forces cannot hold the object together any more and it breaks apart in a manner determined by the structure of the object (whether it's more rigid or more fluid-like) and the types of forces holding it together.

This effect is observable in many places throughout the universe: it is why the earth is wider around the equator than around the poles, why some fast-spinning stars are even more deformed, why many galaxies tend to form flat-ish discs, and why the planets of the solar system lie roughly in a plane.

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A simple analogy is a ball on a string moving in a circle. In the inertial frame the centripetal acceleration is provided by the tension in the string. As the spin ($ \omega $) increases the required centripetal force increases and the tension on the string increases. Too rapid a spin will result in the internal stresses in the string being too great and the string will break. Viewed from a non-inertial frame fixed on the ball, the centrifugal force equals the tension in the string; increasing the spin increases the centrifugal force eventually breaking the string.

For the spinning ball, the internal stresses in the apple are analogous to the tension in the string; too rapid a spin exceeds the allowable stresses in the apple and it "breaks".

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