Weight of Material at water

The surface of the water can deform and will act as a shock absorber. Sand , plasticine, sponge etc will have a similar effect.

At high speed water is also hard and will break some objects. Have a look at a speed boat accident.

A thin walled glass ornament may also break on water.

What object did you use?

Dear Sir,

If we considered stone as material.What induces the stone at air fall fast & what phenomenon happen when it is thrown in water.Please explain.

It sounds like you're talking about a stone falling through air as compared to a stone falling through water. Is that correct? If so, the drag will be be significantly different.

Water is more dense then air so the drag it places on the object keeps it velocity lower. The velocity at which the object impacts the bottom in water is not great enough to shatter the object.

An object falling through air will reach a terminal velocity when the downward force of gravity equal the upward force of drag.

have a look here. http://en.wikipedia.org/wiki/Terminal_velocity

Because water is denser than air the drag in water will be greater and the terminal velocity will be slower.

Also think about the drag changing as the shuttle reenter the atmosphere.

The object will also ¨float" because of the weight of the water displaced must be taken in account.

The object is falling thrugh air both times. It hits the ground on one test and breaks and hits water on the second test but does not break.

Not to be picky, I humbly submit two comments to a wonderfully detailed response.

1. Your comment was that water was 'soft . . .which absorbs some of the energy of the object' . Please note that both the water and the dirt 'absorb' all of the inertial energy since the object stops falling and comes to rest. The main difference is how fast this occurs.

2. The question seemed to me to indicate that the dropping involved was by hand, or at least close enough to the ground that the issue of terminal velocity is probably not realistic to consider, though I was impressed with it.

I posted a response with greater detail further along in this thread.

Respectfully, CJ

Hello CJMcGill:

I thought you post was pretty clear and for that I commend you.

And, yes, I was being picky!

It is an interesting subject though?

I read somewhere, not on here I don't think, a question like this:

If a fly hits a train windscreen when the train is moving at a 100 Kl hour would it stop the train far a nano second. And it is worth a thought for sure.

By the way, what went though the fly's mind as it hit the screen?

Take care.............

Answer; his bee-hind.

And yes, though very miniscule there would be a calculable lessening of the speed of the train, but not a stopping for a moment.

Hello CJMcGill:

Answer; his bee-hind.

And yes, though very miniscule there would be a calculable lessening of the speed of the train, but not a stopping for a moment.

YES! Spot on Sir!..................And it was decided that the windscreen would flex and pretty much take all the 'force'. Though of course, it was theoretical and not something you could measure?

Thanks for the reply.

Take care.....................

Let's call a pillow very soft, in it's ability to slow and object gently. We'll call water moderately soft. Lets' call dry sand moderate, packed dirt moderately hard and concrete hard.

Some objects (such as a large spindly glass structure) will break on contact with a pillow. Some objects, such as a boat dropped from 50', will break upon contact with water. Some objects, such as a bb dropped from 100' will not break even when dropped on concrete. So there is a balancing act between material strength, dropping speed, and surface material.

The terminal speed of a rock is quite high in air, and quite low in water. Therefore, at the surface, the rock must slow very quickly. This slowing is called an acceleration. F=MA, meaning that Force = Mass x Acceleration. The force will therefore be high if the acceleration is high (for a given mass). If the force is high enough, and the falling item weak enough, the falling item will break. Airplanes, when they hit the water, break apart. Rocks so not.

Assuming the "weighted material" you speak of is a chunk of solid or liquid, perhaps the term that these answers are trying to find is elasticity. The collision (of solid or liquid) with water could be termed, elastic...in that, because because water molecules are very widely spaced, in perpetual motion, and free to move, then kinetic energy transferred from the falling object at impact is accordingly transferred to the water over a large volume and a relatively large time span.

Conversely, the constituent particles at the surface of solid ground are tightly bound and/or (in the case of soil) so closely arranged that friction prevents their motion to a degree sufficient to allow kinetic energy transfer except over a (relatively) very small range from the point of impact.

UG

You have, then, a situation in which, because the kinetic energy cannot be fully transferred to the ground (as was the case with impacted water), the kinetic energy that would have been transferred (say, in an elastic collision with water) instead dissipates within the falling, solid (or liquid) body...such dissipation which becomes sufficient to break the chemical bonds which had held the solid body together up until the time of collision. Thus, the inelastic collision results in shattering of the solid falling object.

Dear Guest, At great length the physics of falling objects has been detailed, and very well I might add.

So to be concise, the critical difference is the rate of change.

More specifically your question requires the physics at point of impact. It is said about automobile accidents 'speed does not kill, it is the difference in speed (the sudden stop) that kills".

It has been duly noted how drag differs in air and water. When the falling object is subjected to a drastic and sudden change in the drag co-efficient, such as at the point of contact with water, soil, concrete, or even a vacuum or denser layer of gas, then inertial energy is exchanged.

The issue is the ability of the object to withstand receiving or dispersing energy, in this example the same or more energy (higher drop over water), in the time given for it to occur. Since the surface tension of the water is less than the surface tension of dirt, the time over which the gravitational inertia transfers is different. The distance traveled after contact with each surface is different. Depending on several factors the object will probably travel further past the surface of the water after contact than solid ground.

Calculating the difference in time and distance, though apparently small, from the point of contact to the point the energy change is complete, times the mass, will give the critical detail you are seeking.

This principle also can be illustrated by throwing an egg or water balloon by hand. If the time for the energy transfer from hand to egg is greater than the egg can withstand, things will get messy. On the other hand, a bus full of people that is slowed to a stop over a proper length of time by the brakes can withstand it better than if slowed more quickly, say by a tree.

Hello CJMcGill:

I understand what you are saying,. I was also expecting you to make a point which I have not noticed if it was made.

If an object is fragile which means it deforms less than the speed taken to slow it to a stop, it will break.

It matters not what surface a plastic ball falls onto, it will mostly absorb that contact for deformation.

The only thing to deform, where a hard object in dropped into water, is the water, thus absorbing some of the energy.

No insult implied to you OK.

Take care...................

No insult taken, thanks.

Again however you say the water absorbs some of the energy. In fact it absorbs all of the given energy or the object would continue in motion.

Energy is not created or lost, just transformed, transferred etc. The water tension applies equal and opposite energy to the direction of travel of the object in order to stop it. 'Absorbing' the energy means there will be waves through the water, reflected waves of energy into the object, heat, light etc, all equalling the inertial energy that had the object in motion in order to stop it's motion. The 'strength' of the object is determined by the ammount of energy required to break it. Therefore determining the level of reflected energy required to break the object will be measured in terms of mass X speed. The mass does not significantly change but the speed (which involves time, mps, kps etc.) is the variable. At some speed even the plastic ball will break when hitting water.

The parameters of guest's question reveal that the energy reflected when hitting ground was greater than the energy when hitting water, enough to make the difference in breaking or not breaking. The variable in his equation is not the mass or speed of the object in motion, but the time variable of the stopping distance. This time variable is affected by the density of the stopping material, the viscosity etc, but either way the variable in the energy formula is time. Dirt stops an object much more quickly. Same formula, different time element, different result.

It all comes down to the rate of change. The delta, and the strength of the object.

What he's trying strenuously to say, is that it breaks down to being a matter of the properties (at atomic level) of the colliding bodies...all else being secondary. To essay an explanation based primarily on dynamics such as absorption is, in effect, to answer the (OP guest's) question by begging the question--a fallacious line of reasoning.

That is to say, what Guest essentially asks is, why is the energy release in a collision between solid and water (likely to be) absorbed (with "damage" only to the water), whereas the energy release (and absorption!!) in a collision between solid and solid will result in damage (in loss of structural integrity) to at least one of the colliding solids. Such a question can only be rationally addressed with primary reference to colliding body properties, both physical and chemical. As a simple example...

Due to its own properties and those of water, Titanic was able on the one hand to collide at high speed with water over both instantaneous and extended time frames with no damage to its own, solid self. One the other hand, with only a change in the properties of water, the collision energy absorbed by the ship was sufficient to damage it severely in a very short period of time. (Curiously, when the collision resulted in least disintegration of the ship, the disruption to the water was greatest; when "damage to water" was greatest, disruption of ship integrity was least...amounting only to "normal" wear and tear on the hull and hull paint.)

PS: Note to admin/Mark: the word, disintegration, is missing from the spell check vocabulary.

without going into terminal velocity0 if we look like this ?

The material hits the water surface-

the impulse is till the surface of water (held by surface tension) breaks - in the micro seconds that it breaks - if the force exerted is more than the cohesive force of the material - it will break on hitting even water -

If it is not, as soon as the surface breaks, it gets the archemedis float and the velocity gradually reduces with no more chance of breakage.

In case of hard surface or even dust, the surface will not break, only displace or compress and the impulse transmitted back will likely to be very high compared to liquid and then the chances of breakage will automatically be higher (some times of course we do not break our legs by falling on sand ) and air crafts degenerate - do not powder by hitting the sea