Rock in the rowboat
A rowboat is floating in a swimming pool. Which will raise the water level more - dropping a rock into the water or putting the same rock into the boat? Or does it make any difference?
Labels: logic, outside-the-box
posted by Rajesh Lal at
8:05 AM
38 Comments:
It will be placing the rock into the boat as the displacement of water will be more with the boat as the boat has higher volume than the rock, and then putting the rock into the boat will increase the weight pushing more water out due to weight and volume
of course is dropping a rock into the water.. Happy Chinese New Year for all the chinese in the woRLD... :p
I agree with Zack.
If the rock is say, 2 lbs, it is more dense than the water so an equal volume of water would weigh say, 1.5 lbs.
Dropping the rock in the water would displace 1.5 lbs water. Putting the rock in the boat would displace the full 2 lbs of water.
The exception would be if you really heaved it and were able to splash out more than .5 lbs of water.
I think putting the rock in the boat will bringn the water level up more than putting the rock in the water by its self. When you put the rock in the water alone, the water will only rise by the amount of the volume the rock takes up. When you put the rock in the boat, the weight of the rock is distributed in all the boat and will push the entier boat down, bring the water level up more significantly than the rock on it's own...i hope that makes sense O_o
placing a rock into the boat woud displace more water thus raising it more...
gbusman & Melanie were mostly right... assuming the rock sinks. If the rock sinks it is displacing less water than it weighs (otherwise it would float). In the floating rowboat it would displace a volume equal to it's weight not it's volume which is more than if the rock sinks. The problem states that the rowboat is floating but the boat could be at equalibrum where adding any weight to it would sink it. Dropping a rock (like pumice) that floats would be the same as floating it in the boat. The rock could also be larger than the pool so only a portion would sink (e.g the pool is 3 inchs deep) In which case the boat could displace more than the rock.
in theory putting the rock into the water should displace more, because it would be displacing it's entire volume, whereas, in the boat, it would not be completely immersed, and therefore not displacing its entire volume.
right?
But what if the rock is so big that it sinks the boat and therefore displacing the weight of the boat and the water?
But what if the rock is so big that it sinks the boat and therefore displacing the weight of the boat and the water?
But what if the rock is so big that it sinks the boat and therefore displacing the weight of the boat and the water?
Due to buoyancy, dropping the rock in the boat would displace more water.
The water level will be same in both the cases.
id b puttin the rock in the boat
as the extra weigt in the boat would raise the level .... yea!
When the rock in dropped in the water, it is bound to displace more water. Due to the buoyancy of the boat, the rock cannot displace as much water when just dropped in it!!!
putting the rock in the boat would cause a greater water level increase because the boat has a greater surface area.
greater suface area or not, it won't displace the entire volume of the rock, because it's not immersed, if it's heavy enough to sink the boat, then yes, dropping it in the boat will be better
A lot of you are forgetting about buoyancy. If you drop the rock in, it will not displace as much water as it would if it was in a boat. Why does the boat float? Buoyancy. Not saying the rock will float, just saying it will not displace as much.Its basic physics really. If you were in the water, would you raise the water level more than if you were in a boat? You wouldn't. Nor would they be equal.
Buoyancy has nothing to do with "amount of water displaced"
- A Boat (float) displace X amount of water.
- If I crumpled this Boat, it will sink and displace the same X amount of water.
- The difference is the Area of the displaced water. the larger the area, the higher the upward pressure (thus afloat).
- The displaced amount of water depend on the density/ specific gravity. the higher the SG, the more water is displaced.
Hi,
Call putting rock directly into water RIW and putting rock into boat RIB.
I. If the rock+boat system still floats, then RIB will increase the height of the water in the pool more then RIW.
II. If the rock+boat system sinks, then the opposite is true.
*All this assumes density of rock > density of pool water.
For I.:
Increase to the height of pool water = AdditionalVol_displaced/PoolArea
RIW gives an additional displacement = Volume of rock (Vr).
RIB gives additional displacement = Vs
where Vs = (Vr)(density_rock)/density_water (by equating the buoyant force to the gravitational force on boat+rock system, since it floats)
For II.:
The assumption is that the volume of the boat material itself (since it sinks entirely with the rock) will be far less than the volume of the boat submerged in water when it was floating.
This case happens when the extra cavity in the boat (the space in the boat above the water surface when it is floating) is < (Vr)(density_rock)/density_water...if we consider traditional ideas of rock and boat, then not likely.
That's my take.
me thinks
if rock heavy (enough 2 sink) but small
then in boat will displace more water
weight offset by buoyancy will still out do small volume(size)
but
if rock heavy and big with it
then in water will displace more water
weight / buoyancy won`t out do large
volume (size)
I think that putting the rock in the boat would make the water level rise more because you're adding weight to widely distributed space rather than dropping something not as wide into water which wouldn't have that much of an effect.
Makes no difference
Jacquelyn is correct. From fluid mechanics and the definition of floating and buoyancy; an object floats when force exerted by the displaced water (Density of water x g x Volume of water displaced) is equal to the weight of the object (mass x g). If an object sinks then the force exerted by the displaced water is less than the weight of the object.
I shall assume that the rock sinks in water (most rocks do) and the boat does not sink when the rock is added (otherwise it's a lousy boat).
Let density of water = p, gravity = g, area of pool = A, change in height when rock is in boat = Hb, change in height when rock is in water = Hw, mass of the boat = m1, and the mass of the boat and rock = m2.
The change in weight of the boat when rock is added IS EQUAL to the change in the force exerted by the water (since it is floating) therefore m2-m1 = pgA(Hb).
But, m2-m1 = the mass of the rock = mR.
In the second case, mR IS GREATER THAN the change in force exerted (since is sinks) therefore mR>pgA(Hw)
Equating mR gives pgA(Hb)>pgA(Hw), therefore Hb>Hw.
Thus the water level increases more when the rock is added to the boat, providing the rock sinks in water and the boat is not sunk by the rock.
(NB: if the rock is neutrally buoyant, it makes no difference where it is added)
It doesn't matter. No matter what, the water will rise the same amount.
i say we put the rock in the water and it floats so i put it into the boat nstead
It would be putting the rock in the boat, but both the boat and the rock would have to be a certain size as the size of the swimming pool is not stated. So actually without the size of the swimming pool the answer is inconclusive.
how many of you have ever seen a floating rock???
oh and putting the rock in the boat would displace more water, causing the water to rise up the edge of the boat a fraction, whilst the boat would be lowered into the water a fraction.
but if the rock is big enough, it could cause the boat to sink, in which case the water level would raise so high it fills the boat a bit lowering the boat, displacing more water, untill it over flows and the boat and rock both go under which would not raise the water level all that much, but would still raise it.
if u understood that, well done, even i got confused.....
i say putting it in the boat would raise the water more......i did this in year 5 science
displacement of water is affected by the weight of an object and it's volume.
if 2 items have the same volume, but different weights (e.g the boat with the rock, and the boat without the rock) the heavier item will displace more water.
but if 2 objects weighed the same but had different volumes (e.g the boat and a rock of equal weight) the largest volume would displace more water.
so if the boat would displace more water than the rock, and the boat with a rock in it would displace more than the empty one, i am to assume, the boat with a rock in it would displace more water than the rock alone.
and seeing as though, when water is displaced it has to go somewhere (in this case up) this would raise the water level.
thanks :D
It depends on the density of the rock. If it's small and extreamly heavy, putting it in the boat will raise the water level more. If it's large and light, then putting it in the water will display more, as it would not exert much force on the boat.
But, as the question is asked, there are too many variables that are not defined to properly answer the question. how big is the boat? will the rock sink the boat? does the rock float? etc..
Putting the rock int he water is a direct displacment by volume. Putting the rock in the rock in the baot varies teh weith therefore dispalcement of the water under the surface of teh boat, so statictically it is impossible to answer this question. Unless you have rock which would displace the same amont of water, in the water as its weight would incure in the boat
So their is now answer
the water volume will raise if you put the rock in the boat, you are adding more mass to the water by doing so. this will include the boat and the rock in the water, rather than just the rock.
I read this is in science trick book thingy, it really is putting the rock in the water.
it would make the water level rise higher if you dropped the rock in the boat while the boat was in the water. simple physics.
Has anyone said how big this rock is? And anyways, if it is just a pebble, then it wouldn't make a difference, or not enough of one, b/c of the bouyancy of the rowboat.
Trick question.
WAY too many variables to consider:
How sharp is the rock? sharp enough to poke a hole in the boat?
How heavy is the rock? will it make the boat sink?
How dense is the rock? will it float?
How cheap was the boat? Will it break easily?
How much of the pool is filled? filled to the brink? enough so that a tiny amount of displacement will overflow and the space that was taken up makes water push back up to the brink?
How strong did the rock get put in there?
I feel bored now, so i will go check other puzzles.
if you want to add any other variables, go ahead.
to avoid mix-ups, call me
"ghostrider655" as i dont have a google account
I agree with soccerchck044 there is no difference
I would think this is an issue of weight versus volume. Dropping something in the water displaces the volume in water, where as the weight of the object decides how much the boat is depressed and displaces water.
Imagine a tiny rock that weighs 500000000 tons. If you drop this in the pool, it will displace the marble's size of water. If you put it in the boat. The boat will get pushed down quite a bit, displacing a lot of water.
ANSWER
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A Rock in the Boat displaces WATER equal to its weight
where as a a rock inside water displaces Water equal to its volume.
Since the Rock is heavier than an equal volume of water, putting the boat will raise the water level more
GOT RIGHT
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Zack Redstar, gbusman, Melanie, fun physics, Jacquelyn, Silver Blur, Sparkle, nugget523, and few anonymous guys
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