Floating Leaves
Jim falls onto a net covered in leaves. The video reveals the mysterious motion of the leaves, which appear to stay in the air while the net below them stretches downwards.
Why don't the leaves fall along with the net?
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11 solutions
if the person and leaves were falling in a vacuum, they would fall at the same rate (because there is no air resistance) in space, or would the body fall faster due to its mass?
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The body would fall faster, but not because of its mass. By the time the body reaches the net, it already has some speed, whereas the leaves are at rest. They all start accelerating with g . Since the leaves are starting from zero speed, they would move slower than the body.
No the result would be different
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Could you please tell us what you have in mind?
It actually depends on the velocity with which he falls if greater than acc due to gravity then yeah if not then no
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How would you compare velocity with acceleration? They have different units. Acceleration is the rate of change of velocity.
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By that i meant acceleration not velocity. And it was not he it was the net. Sorry friends
@Pranshu Gaba ,is absolutely right, Velocity of an object is the speed of the object and the direction of its motion. However, acceleration is the change in velocity divided by time it takes for the change to occur.
velocity can not be compared with acceleration.
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By that i meant acceleration not velocity. And it was not he it was the net. Sorry friends
I meant the velocity of the net in a second of acceleration
I think guess and check works best. I thought it was air resistance because in the picture I saw the leaves that where falling last, I thought it was because they where lighter. My question about this problem is, How does Inertia work?
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E Gapp's solution explains this well. Inertia is the tendency of a stationary object to remain at rest. When the net was suddenly taken away from underneath the leaves, inertia caused the leaves to remain at rest momentarily after Jim fell. After that, the leaves started to fall because of gravity.
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Actually, inertia caused the leaves to want to stay at rest. A small, but crucial, difference. Gravity's effect was instantaneous--those leaves were moving immediately. We just couldn't see much acceleration due to only witnessing maybe a tenth of a second's acceleration. Having so little mass, leaves have very little inertia. I.e., it takes very little force to accelerate them, or to hold them still in the face of gravity. The best answer is air resistance.
This would have been a good illustration of inertia if the actual correct answer wouldn't have been included and deemed wrong. Or if we were instructed to think of this in a theoretical sense and told to ignore the instant acceleration present, though not terribly visible, in the reality of the situation.
So if this was in a vacuum you would see the same result?
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The possibility of an extrinsic origin for inertial reaction forces has recently seen increased attention in the physical literature. Among theories of extrinsic inertia, the two considered by the current work are the hypothesis that inertia is a result of gravitational interactions and the hypothesis that inertial reaction forces arise from the interaction of material particles with local fluctuations of the quantum vacuum.
as,no external force is working,so there will be a different result for vacuum.both will fall at the same time.
anyway, Galileo's famous Pisa Tower experiment proves that,if there is no external force--the mass will create no change to maintain its inertia.
Yes, the same result will occur even in the vacuum.
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how?please,give an explanation.
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@Mohammad Khaza – I think I understand it now based on Zain's solution. The leaves will fall due to gravity but they are starting at zero speed and will accelerate at mg. The acceleration of the net is much greater due to the impact of the falling body so the net leaves the leaves behind. Air resistance will slow the leaves' fall a bit but removing the air resistance (in a vacuum) will not make up for the difference in acceleration.
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@David Winson – that's great.i have now a clear idea.
Relevant wiki: Newton's First Law
Jim does not affect the motion of the leaves, so due to inertia, the leaves would tend to stay in place. Eventually, the leaves gain speed as gravity accelerates them. However, gravity's pull on the leaves is not as strong as Jim's force on the net, so the leaves briefly "hover" above the net.
Why do you say that the gravity is not as much as Jim force?
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It isn't really force, rather momentum. When Jim hits the net, some of his downward momentum is transferred to the net, causing it to be pushed down more than it would be by gravity acting on the net alone. That said, it is a combination of forces effecting the transfer of momentum. The force Jim exerts on the net (and hence vice versa) and how they change over the time interval depend on a number of factors, including his momentum just before the impact and the elasticity of the net.
I like the use of "hover", it's appropriate. I considered that the Law of lift supercedes the Law of gravity or that the leaves were just caught off guard!!
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The leaves were in equilibrium before Jim fell. That is, gravity balanced the buoyant force and the normal reaction from the net. When Jim fell, the normal force suddenly became zero. Because of inertia, the leaves remained at rest for an instant after Jim fell. This explains why the leaves don't fall along with the net.
By the way, the buoyant force in air is very weak because the density of air is very small.
Relevant wiki: Newton's First Law
The falling person's body doesn't actually affect the leaves very much, but it pushes the net down very quickly. This is roughly equivalent to a situation in which the net magically vanishes and the leaves fall under the influence of gravity.
While it is true that inertia does indeed have an effect on the leaves, since everything with mass has inertia, I don't believe it is correct to simply rule out air resistance as being a correct answer. Since the leaves have a much greater surface area compared to their mass, they present much more air resistance than Jim, and the net do. And, since the net has holes in it, air is able to move through the net, or more accurately, the net is able to move through the air. Since it is all relativity, this is basically the same thing. Thus the leaves do experience a noticeable effect due to air resistance. If this experiment were reproduced in a vacuum, then yes, inertia would be entirely responsible for the effect, but air resistance cannot simply be ruled out in this instance.
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You're right, but with multiple choice, go with the "best answer", which is inertia
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The best answer actually is air resistance. Inertia only prevents an object from moving when there is no net force acting on it. Yeah, there is resistance to movement, but it doesn't prevent movement in the face of an unbalanced force (gravity), even for an instant. Intuitively you could say that inertia is the reason for gradual acceleration as opposed to instaneous terminal velocity.
Those leaves were moving the picosecond they were no longer in contact with the net. The video is simply misleading and obviously confused people (you can tell from a couple of the comments). This isn't teaching science.
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@Richard Knudsen – The key phrase is "along with the net". The leaves take time to speed up under the influence of gravity (due to their inertia), and therefore do not keep up with the net. I'm not disputing the fact that the leaves fall, or that air resistance plays a role. However, this effect would still happen if there was no air at all.
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@Steven Chase – I agree, you absolutely would see the same effect in a vacuum, but the acceleration would be more rapid--probably visibly so. So I still contend that air resistance had the greater impact in the example and is therefore the most correct answer.
My big problem is that the sloppy wording of the question is leading people to believe that the Looney Tunes Effect is real and knowledgeable people are reinforcing the idea by saying they do indeed float for an instant.
I wrote that ^^^^^^^^^^ and then went back to the question because you reminded me of the wording ("along with the net"). I admit that I processed the question to mean, basically, "why did the net fall but the leaves didn't?" (but lots of people did, hence the idea being posted that they briefly remained at rest after the net dropped), when they actually meant, "why didn't the leaves fall at the same rate as the net?"
So my mistake, BUT, that meaning makes it even worse! The actual correct answer is the inertia (rather, momentum) of Jim. He's acting on the net, causing it to drop in height very rapidly. He's not acting on the leaves. Only gravity is doing that, with air resistance pushing back (and yes, the property of inertia making them accelerate slowly whether in a vacuum or not).
So the leaves didn't fall along with the net, because the net had a far greater combination of forces acting upon it than the leaves did.
BOOM! Brilliant is STILL wrong!
:)
EDIT: fixed a couple typos.
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@Richard Knudsen – Ok, now I understand your concern about the "Looney Tunes" effect, in relation to the wording. More learned folks will know that this is just poetic license, for effect. However, there is some potential to mislead. I think we're in violent agreement :)
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@Steven Chase – Have a good weekend.
The confusing aspect of this question is that the "floating" effect is indeed accentuated by air resistance. Had there been marbles on the net instead of leaves, some of them would have "floated", too, but not as dramatically.
Sheesh. All of the above are wrong--the correct answer is that they instantly started to move due to removal of the balancing normal force which allowed the still active gravitational force to get its way. Inertia and air resistance keep them, initially, from moving quickly--they want to stay at rest due to inertia and accelerate very slowly due to air resistance. Plus the special effect known as slow motion video simply makes it appear they stayed still.
Way to confuse people new to, but potentially interested in, physics, Brilliant.
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I think Steven meant the same. The leaves start to move by acceleration g whereas the net moves down much quicker leaving the leaves to lag behind.
Often the slow-motion videos are captured to understand the fast-moving phenomenons, so the video is a plus for me. At the real rate, we may not be able to appreciate the effect of inertia much.
@Steven Chase Sir , My question can be irrelevant to your explanation however, i will to know what would happen if mass of the leaves is comparatively equal to that of Jim. :)
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if their mass is same,i don't think same inertia will work for them.
cz,when Jim is falling on the leaves,suddenly a very powerful force working on the leaves which is greater than the gravitational force of leaves.
if leaves and Jim both have to fall at the same time,then Jim's weight must be less than leaves.
Leaves aren't very dense. Hence, an equal mass of leaves would form a pretty thick layer. The thickness of the layer (and the resulting interaction between Jim and the leaves) would prohibit Jim from passing through "unnoticed". Many of the leaves would be seriously disturbed as a result, and would probably be dragged downward with him.
Is the same principle involved when you pull a tablecloth out from under a set of dishes without upsetting them?
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Insofar as both effects result from the inertia of objects initially at rest, yes.
@Richard Knudsen - "they WANT to stay at rest!?" They are not sentient beings.
This is not a solution. I am bothered by the categorization of this puzzle as 'basic'. I did get it right, though I had to think about it for while - I didn't think 'Oh that's easy' ..... . And I think that I'm quite a clever person - not a dunce! Do others think this was 'as easy as ABC' or am I right to be 'a little bit miffed'? Regards, David
I hear you. Leaves are easily moved around by even the slightest movement of air. The trampoline is made of a mesh which would let some air through. But as the trampoline moves down so fast it must draw some air down with it. That would create a downwards force acting on the leaves. On the other hand, there are so many leaves on the trampoline that they probably act together as a single larger object and are less affected by this air movement. Anyway, this is all beyond the scope of the question. It is supposed to be a simple question about inertia.
What's easy for you might be hard for some other people, and vice versa. If you struggled at this problem, that's okay, it means you're learning. Is important to have a growth mindset. Cheers, Eileen.
I also got it right but believe that the question is flawed. The buoyancy of leaves in air is admittedley very little and for most practical purposes can be ignored - but theoretically it exists. Air resistance will have an effect which will increase as the leaves gather speed.
Another point is that Jim's acceleration under gravity began at some distance above the net; the leaves start to accelerate only when the net is moved out of the way by Jim's fall i.e. the net is no longer exerting the same magnitude of force on the leaves to counterbalance gravity. One could also consider leaf on leaf forces as the layer of leaves is more than one leaf thick, etc, etc.
Inertia is right, but at almost the same level, in a different point of view, air resistance is what causing this. With gravity present and no resistance, they would move!
Point here is that at the BEGINNING inertia is the main factor; LATER once the leaves start gathering falling speed the air resistance becomes more and more a factor.
When Jim is falling, there is more weight when he hits the net due to his speed, but the net and the leaves are at rest. His body hitting some of the leaves and the net made them go down with him, as you can see from the video. The rest of the leaves stayed at rest, due to inertia, for a moment, but soon the gravity pulled them downwards, so they began to fall.
See a fully-extended slinky drop video.
The bottom of the slinky seems to 'levitate' in the air with the tension until the top of the slinky descends to that point.
Inertia: a force that slows down the speed of one object in motion. The leaves are falling at a much slower pace than Jim's body is. That's what makes it look like he's going through the net at first. That would be a SERIOUSLY sick jump though!
if it happens in a place where no external force works,then what will happen ?
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Hmm... I'm not sure, but I'd imagine they would go at the same speed. I could be wrong, though.
the position of the leaves, also leaves are flat.So if you let lots of leaves fall at once they will fall slower. and if the net is falling so quickly they will take even longer.
According to newton 1st law body wants to be in continues in its state of rest or uniform motion here rest.
The answer could be due to inertia of the leaves, buoyant force (byoiancy for solids) of leaves in air ( Complified situation and with proper conditions being noted and calculated if done so ) , or. Due to air resistance. The first option and last option was right whereas second was with respect to air. But should have been with leaves in air. So the mistake lies in the question too
~Mistake -
•Velocity of falling not given
•the location of happening is not given.
•the necessary conditions are not given
•the given conditions are only the conditions is suitable for human. i.e. Air is present........... Amnd the other factors in the question
Till the velocity of falling is less than or equal to acceleration due to gravity to the centre of gravity
If it's greater then the last becomes unsuitable so first one will be correct. Also second is if it was buoyancy of leaves. (Same conditions) in air. Of the respective region
It also depends on the height from which he falls
Note. The are cannot be earth also
Guys email any doubts to janesh262@gmail.com. Also say if wrong
The question is incorrectly phrased. The leaves do in fact fall just as the net does - however the net accelerates faster than the leaves so it appears that the leaves are not falling. The net is "falling" faster than the acceleration of gravity while the leaves are falling at the rate of gravity. Because the video is slowed down it appears that the leaves are not falling when they will have begun to fall as soon as the net is moved away below them - this appearance of the leaves not falling is an incorrect perception of what is actually happening.
Relevant wiki: Newton's First Law
Inertia is an inherent property of an object to resist any change in its state of rest or of uniform motion. All objects continue its state of rest or of uniform motion along a straight line unless it is acted upon by an external force. This tendency of objects to continue its state of rest or to keep moving with the same velocity is called inertia.
example----- Fruits fall down due to inertia of rest when the branches of a tree are shaken. Fruits and branches are both at rest, but when branches of trees are shaken, branches starts moving where as fruits remain its state of rest and so separated from the branches and fall down.
Dust particles on a carpet fall if we beat the carpet with a stick is another example for the inertia at rest. When we beat the carpet with a stick carpet starts moving, but the dust particles remains at its state of rest and separated from the carpet.
With a quick pull, a table cloth can be removed from a dining table without disturbing dishes on it due to the Inertia of rest. The inertia of rest of the dishes keeps them where they are.