today's physics brain teaser
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today's physics brain teaser
Ok, here is today’s puzzle
Imagine a boat floating on a small pond. The boat is loaded to maximum capacity with large rocks. You measured the depth of the water and find it to be exactly 4 feet deep. If you throw the rocks overboard what effect would it have on the depth of the water and why?
Imagine a boat floating on a small pond. The boat is loaded to maximum capacity with large rocks. You measured the depth of the water and find it to be exactly 4 feet deep. If you throw the rocks overboard what effect would it have on the depth of the water and why?
Last edited by sigma; Dec 9, 2005 at 11:22 AM.
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From: Oak Harbor, OH
none, the displacement would stay the same, the boat would have less mass afterwards, but the rocks would counterbalance it. Now if you'd put the rocks on shore, the lake would have fallen, a very small amount, but none the less it would have fallen.
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From: just outside of Phila.
The water will get "deeper". If you don't throw any rocks on the spot of the original measurement, the boat will rise as more weight is removed, and your measurement will be altered..................right?
I don't think I know as much about physics as I like to believe.
I don't think I know as much about physics as I like to believe.
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From: Oak Harbor, OH
The boat will rise yes, because the boats displacement has changed. If you dump all the rocks off in the same spot they then have the same mass as when they were in the boat. The boat plus you plus the rocks (now at the bottom) are all the same mass, the water level will stay the same.
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From: colorado
providing the sides of the pond go straight up and the rocks are evenly distributed on the bottom the depth will stay the same because the displacement of the rocks and the boat havent changed there are just in different places
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From: Peoria, IL
This is another problem I was wrong about yesterday. The level of the lake will fall.
An object floats when it is able to displace an equal mass of the matter it is resting in. Therefore in order for the boat to float with the rocks inside it would have to displace as much water at the mass of the boat and rocks combined.
When the rocks are thrown overboard the boat has to displace less water in order for it to float, so it will rise in the water. The rocks however will not float. Therefore they will only displace as much water as it would take to fill an equal volume, not equal mass. Since 1kg of rock would take up much less volume than 1kg of water (which is what it displaced when the rock was floating in the boat) the level of the lake will lower.
An object floats when it is able to displace an equal mass of the matter it is resting in. Therefore in order for the boat to float with the rocks inside it would have to displace as much water at the mass of the boat and rocks combined.
When the rocks are thrown overboard the boat has to displace less water in order for it to float, so it will rise in the water. The rocks however will not float. Therefore they will only displace as much water as it would take to fill an equal volume, not equal mass. Since 1kg of rock would take up much less volume than 1kg of water (which is what it displaced when the rock was floating in the boat) the level of the lake will lower.
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We have a winner! BeerStalker is absolutly correct. When the rocks are in the boat they displace a volume of water equal to their weight. When the rocks are thrown overboard they displace a volume of water equal to their own volume. Since the density of rock is much higher than water the pond level would fall by taking the rocks out of the boat and placing them in the pond.
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From: colorado
Originally Posted by sigma
We have a winner! BeerStalker is absolutly correct. When the rocks are in the boat they displace a volume of water equal to their weight. When the rocks are thrown overboard they displace a volume of water equal to their own volume. Since the density of rock is much higher than water the pond level would fall by taking the rocks out of the boat and placing them in the pond.
this picture has 4 quarters in a small plastic cup floating in a cup of water (dyed red) the water is level with the tape.
here the quarters have been taken out of the plastic cup a placed in the water. the plastic cup is still floating in the water but its hard to see.
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From: Davenport, IA
Originally Posted by 69 and 85
I tried the same thing... only with beer as I had no red dye. Funny thing is every time I tried it, the cup was always empty when I spit out the quarters.
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From: KCMO
phloating physics
Similar question. A glass of water, containing a large ice cube, is filled to the brim, such that one more drop will make it overflow. Like an iceberg, most of the ice is beneath the surface, but part of it is above. When the ice melts, what will happen to to the water level?