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So I saw this tip enter image description here

but I don't think this is true, it would be that your leg or shoe is more flexible than a hard floor so the momentum change would be slower right?

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    $\begingroup$ Think of a football player (soccer player in the US, Canada, & Australia). They are taught from a young age to develop "touch", to be able to catch the ball with their foot so gently that the catch wouldn't break an egg. Eventually some develop such fine touch that they can catch an egg with their foot. $\endgroup$ – David Hammen Jan 4 '15 at 14:41
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    $\begingroup$ @DavidHammen "Eventually some develop such fine touch that they can catch an egg with their foot." Really? $\endgroup$ – Keep these mind Jan 4 '15 at 15:29
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    $\begingroup$ Cooks know the same trick when dropping dishes - but don't try this with knives! $\endgroup$ – MSalters Jan 4 '15 at 17:33
  • $\begingroup$ I'm not sure about that but I do put a leg so that phone will hit my leg first and then land on ground relatively slower speed than that of free fall from my hands height 😄 ;) $\endgroup$ – uday Jan 4 '15 at 18:43
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    $\begingroup$ @GlenTheUdderboat - Really! (Well, almost really.) I personally saw demos a couple of decades ago when my kids were young. That was in the midst of a volunteerism phase when I became president of a local soccer club. (1000 parents, a bankrupt uniform vendor, and months of Houston rain made me rethink that.) Soccer in the US was just making a comeback from the old NASL days of glory. Europeans galore who wanted to work and live in the US and train kids on the side showed off their skills. More than one demonstrated catching a dropped egg with a foot. Okay, they used hard-boiled eggs, but hey! $\endgroup$ – David Hammen Jan 5 '15 at 0:10
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You are correct. It's all about reducing the maximum force experienced by the phone. The phone has gained a certain amount of momentum when it gets to the level of the floor, and if it's going to come to a stop, it will lose all that momentum because of a force, either from your shoe or the ground.

This graph illustrates the force your phone would experience over time if it hit the ground (purple curve) or if it hit your shoe (red curve). The momentum change is the same in both cases - that is, if you integrate (add up) all the momentum that the force gives the phone in each case, you get the same momentum change - the phone comes to a stop.

enter image description here

However, in the case where it hits the ground, the ground is, as you said, much less flexible than your shoe, so the ground delivers a very large force, stopping the phone in a tiny period of time. This large force is what shatters your screen. In the case of the shoe, your shoe is nice and soft, so it delivers a smaller force, stopping the phone over a longer period of time. In fact, if you move your foot downward as the phone hits your shoe (known as "trapping" in soccer), you can stretch out that period of time over which the phone comes to a stop even longer, making the maximum force even smaller.

Since the maximum force in the shoe impact is much smaller than the maximum force in the ground impact, the phone is more likely to break if it hits the ground rather than your shoe.

The whole "transferring to lateral motion" thing is a red herring. If it loses all its vertical momentum, then a vertical force was applied to it. The only question is, was it a high force for a short amount of time (bad) or a low force for a longer period of time (good). In fact, if you actually "kick" it, you're irrelevantly adding momentum in the horizontal direction, which just introduces an unnecessary extra force onto your precious phone.

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  • $\begingroup$ Transferring to lateral motion is not a red herring. If you kick the phone appropriately to convert the falling momentum into sideways momentum (for example, if the phone simply hit a small trampoline angled at 45 degrees from vertical), then the phone will be more likely to be scratched (landing with high sideways momentum) but still less likely to have a shattered screen. $\endgroup$ – Sparr Jan 4 '15 at 18:53
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    $\begingroup$ It's incorrect to think of momentum being "transferred" from the vertical to horizontal directions. The individual vector components of momentum are individually conserved. Compare these situations - the phone hits the floor and comes to a stop, or the phone hits an angled surface, and loses all vertical motion (while gaining some horizontal motion). The change in vertical momentum? Identical in both cases. If you kick your phone, you're giving it a vertical impulse to stop its vertical motion, AND giving it a horizontal impulse. Better to just let it hit your foot, rather than kick it. $\endgroup$ – Brionius Jan 4 '15 at 19:08
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    $\begingroup$ Most people aren't good enough with their feet to "let it hit" rather than kicking it. $\endgroup$ – Sparr Jan 4 '15 at 19:43
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    $\begingroup$ @Sparr I would bet that Brionius is right, that the reason kicking a falling phone might save it from damage is that even a kick is able to apply some vertical force. $\endgroup$ – David Z Jan 4 '15 at 20:49
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    $\begingroup$ By increasing the duration of the impulse, we reduce the maximum force on the phone, as you can see in the graphs. $\endgroup$ – Brionius Jan 5 '15 at 23:50
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A second effect is that you are very likely to hit your phone off-centre. As a result, you'll send your phone spinning, but it will continue its fall. Your foot won't have stopped it, just slowed it down, and it will still hit the floor. This means the energy is absorbed in two impacts, not one.

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  • $\begingroup$ This question isn't asking about stopping it, it's asking about it breaking. $\endgroup$ – AStopher Jan 4 '15 at 20:52
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    $\begingroup$ I don't hope I have to explain to the downvoters the joke about the fall not killing you, but the sudden stop at the end. It should be sufficiently obvious that breaking = force = sudden deceleration = sudden decrease in kinetic energy. $\endgroup$ – MSalters Jan 4 '15 at 22:31
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    $\begingroup$ This seems like an incomplete answer to me. Also your answer neglects the possibility that your foot actually increases the speed of the phone (although it will change the direction). $\endgroup$ – Brandon Enright Jan 4 '15 at 22:53
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    $\begingroup$ @BrandonEnright: It's indeed not a stand-alone answer, obviously - it starts with "a second effect". Brionius' answer already covered the other main effect (more time to decelerate). Adding a horizontal speed component rapidly complicates the analyis, though - as the phone hits the ground, friction will slow down the horizontal component, and we have no idea how rapidly it will do so. $\endgroup$ – MSalters Jan 5 '15 at 7:44
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The "shoe" doesn't have to be soft for this to work. Look at the following diagram:

enter image description here

Here, I replace the shoe with a tilted plane. The normal velocity of the phone with respect to this plane is smaller (by $\sqrt{2}$ if the plane is at 45°) and so the force of the impact will be reduced - even if the shoe is made of the same material as the floor. If the impact was elastic, the vertical momentum would indeed be transferred to horizontal momentum, and the final vertical velocity with which the phone hits the ground is equivalent to the drop from the height of the kick.

The same effect would be achieved if you had a less angled plane moving at sufficient velocity - but it would result in greater impact force (unless the plane / shoe was softer).

An extreme example of how well this "glancing impact" can reduce the force of impact can be seen in ski jumping: Although the jumper may drop 50 m or more from the point of take-off, he lands safely because the slope curves down with him - so the relative velocity is once again small (source of image: http://www.shinmai.co.jp/oly-eng/1997/97012304.gif):

enter image description here

You can of course try to "catch" the phone on your foot, counting on the relative softness of the foot/shoe to cushion the blow, but to the extent that you can do anything to reduce the vertical velocity you will indeed improve the chances that it survives the impact. The "outrageous" statement has some truth in it. It is possible to transfer vertical momentum to horizontal momentum, and doing so with the simple tilted plane reduces the maximum force of the impact.

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  • $\begingroup$ I would give you a +1, but the "glancing impact" diagram you drew above shows the vertical momentum disappearing in the impact. If that is the case, then the same vertical impulse is being applied as if it hits the horizontal floor directly. If you edit the diagram to show the phone retaining some downward velocity after the impact, then your point is valid. $\endgroup$ – Brionius Jan 12 '15 at 17:07

protected by Qmechanic Jan 4 '15 at 20:58

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