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15-02-2015, 07:06 PM
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BANNED
Join Date: Jul 2012
Posts: 2,886
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Trying to settle an argument between a few boys.
I have a 500kg rock to move. How much does it weigh when underwater compared to out of water? Is it 500kg minus the weight of the water it displaces? Is it the weight of the water it displaces? We all agree that it weighs less. does anyone know for sure? TIA |
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15-02-2015, 07:15 PM
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Donating Member
 
Join Date: Jan 2005
Location: Morayfield
Posts: 28,159
  
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Archimedes principal. It will weigh less according to the weigh of water it displaces .
Mass remains constant.
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15-02-2015, 07:22 PM
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Former BTIKD
 
Join Date: Mar 2005
Location: Sunny Downtown Wagga Wagga. NSW.
Posts: 53,197
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Quote:
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Dying at your job is natures way of saying that you're in the wrong line of work.
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15-02-2015, 07:23 PM
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FF.Com.Au Hardcore
Join Date: Jun 2010
Posts: 700
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It will weigh more, because it's the weight of the rock plus the weight of all the water above the rock.
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15-02-2015, 07:52 PM
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If it ain't broke........
 Join Date: Dec 2007
Location: Sunshine Coast Qld
Posts: 18,784
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Quote:
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Visitors welcome Relatives by appointment only |
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15-02-2015, 08:40 PM
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Peter Car
Join Date: Dec 2004
Location: geelong
Posts: 23,145
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A 500kg rock is still 500kg no matter where it is?
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15-02-2015, 08:55 PM
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Thailand Specials
Join Date: Aug 2009
Location: Centrefold Lounge
Posts: 49,576
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I stopped taking physics after year 11 for a damn good reason.
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15-02-2015, 08:59 PM
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FF.Com.Au Hardcore
Join Date: Apr 2013
Posts: 3,318
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Does that principle apply to the Dead Sea, which has an extremely high salinity level?
Closest thing I've ever come to science is trying to sabotage my brothers chemistry kit. |
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15-02-2015, 09:08 PM
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#11 | ||
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Donating Member
Join Date: Oct 2013
Location: Wellington NZ
Posts: 11,392
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Upthrust = apparent loss of weight of object = weight in air - weight in liquid. The upthrust is altered by density of the water i.e. saltwater increases the amount displaced.
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15-02-2015, 09:19 PM
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FF.Com.Au Hardcore
Join Date: Dec 2004
Location: Canberra Region
Posts: 9,011
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What altitude?
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15-02-2015, 09:46 PM
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#13 | ||
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Fat 460
Join Date: Jun 2006
Location: Perth
Posts: 2,649
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If it's Pumice stone it will float.
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15-02-2015, 09:56 PM
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FF.Com.Au Hardcore
Join Date: Jul 2006
Posts: 7,331
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Wouldnt a denser, smaller rock displace less water? Assuming neither will float or anything stupid like that?
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15-02-2015, 10:00 PM
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Boss 335
Join Date: Jan 2006
Posts: 4,330
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asking google 'how heavy is a rock' returns:
118 kg Dwayne Johnson, Weight |
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15-02-2015, 10:08 PM
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BANNED
Join Date: Jul 2012
Posts: 2,886
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ROFL some interesting answers here...
sounds like a cubic metre of concrete (2.4T) weighs 1.4T under water. cheers |
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15-02-2015, 10:34 PM
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FF.Com.Au Hardcore
Join Date: Dec 2004
Location: Brisbane
Posts: 1,198
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Exact answer...
=1000 kg/m^3 V = m / ρ V = [500kg] / [1000 kg/m^3] = 0.5 m^3 Volume of displaced water = 0.5m^3
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16-02-2015, 12:05 AM
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FF.Com.Au Hardcore
Join Date: Jan 2005
Posts: 4,167
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Quote:
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16-02-2015, 07:31 AM
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When in doubt, GAS IT!!
Join Date: Sep 2006
Location: Lower Eyre Peninsula, SA
Posts: 3,018
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If it's static I reckon it'd weigh the same 500kg. If you try to lift it, it would take more force or energy to move it as you have to displace to water above it as you lift so it would effectively make it appear heavier than it's 500kg.
Take a brick, suspend it from a set of digital fish scales and take a reading. Then suspend the brick in a bucket of water on the scales and read it again. My money is on it still reading the same once it settles.
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16-02-2015, 07:32 AM
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The one and only
Join Date: Dec 2004
Location: Carrum Downs, Victoria
Posts: 9,053
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Rock isn't as heavy as metal \m/
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16-02-2015, 08:22 AM
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FF.Com.Au Hardcore
Join Date: Jun 2011
Location: Pt Lincoln far side South Oz
Posts: 5,875
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I need a strong drink
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16-02-2015, 09:23 AM
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#24 | ||
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Former BTIKD
Join Date: Mar 2005
Location: Sunny Downtown Wagga Wagga. NSW.
Posts: 53,197
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cs123 got it on the first reply, this should explain it a little .....
http://physics.weber.edu/carroll/arc.../principle.htm 
__________________
Dying at your job is natures way of saying that you're in the wrong line of work.
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16-02-2015, 09:37 AM
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#25 | |||
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FF.Com.Au Hardcore
Join Date: Apr 2012
Posts: 976
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Quote:
except the density of rock is more likely to be 2,500kg.m^-3. Without knowing the type of rock, this is agood enough approximation. Malazn mafia is on to something, though, get Dwayne to move it for you!! Steve |
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16-02-2015, 10:42 AM
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#26 | ||
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Regular Member
Join Date: Jun 2012
Posts: 126
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***WARNING! SCIENCE CONTENT!***
Discounting any inherent buoyancy in the object that would create a notable difference in behaviour between air and water: The mass is constant, with the weight depending on the gravitational force in the rock's environment, ie, it would weigh heaps more on Jupiter than on Earth. On earth, gravity is 9.807m/s^2, so a 500kg rock (or anything else) would require 4903.325 Newtons (N) to lift it, regardless of whether it is water or air. The easiest way to think about this is lifting it off the ground, or the seabed while standing on the solid surface. HOWEVER! if you were using a helicopter versus a submarine, this were it might become 'easier' or 'harder', despite the required force being exactly the same. The only difference is that water is a denser medium, so it will be easier to push against to get up to your ~5000N. The amount of water (by weight) displaced is equal to the weight of the object - if the object floats. If the object sinks, the amount of water displaced (by volume) is equal to the volume of the sunken object. eg. a 2000kg boat will displace 2000kg (2000L) of water, but a cannonball with a volume of 1L, will only displace 1L of water, regardless of weight. |
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16-02-2015, 10:46 AM
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Isn't it obvious?
Join Date: Nov 2009
Location: in a world of idiots
Posts: 5,383
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my brain thingy hurts
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16-02-2015, 08:02 PM
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#29 | ||
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Peter Car
Join Date: Dec 2004
Location: geelong
Posts: 23,145
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Beat it nerds. Your fancy science ain't welcome here
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16-02-2015, 11:29 PM
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#30 | ||
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Formerly ST170ish
Join Date: Jan 2005
Location: Down south
Posts: 1,673
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What if I hit your rock and break it
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My bad attitude escalates in direct proportion to the amount of stupidity I am presented with!!! 
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How heavy is a rock?
Linear Mode
