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Archimedes Principle question 
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#1
Nov2811, 07:26 AM

P: 135

I have found a question that has got to my nerves :
A block of wood of length 40 cm and area of crosssection 15 cm^{2} floats in water with 3/8 of it's length above water. What is the density of wood ? (Density of water  1g/cm^{3}). My attempt to get the solution : Length of wood above water = 3/8 * 40 = 15cm Length of wood below water = (4015)cm = 25cm Now, Upthrust = 375cm^{3}*1*10 = 3750 N Rest, I don't know ! Please help. It's urgent because I am going to face some deadly questions the day after tomorrow. Thank You. 


#2
Nov2811, 07:46 AM

P: 1,195

When an object floats and is motionless, its weight exactly equals the upward thrust. The upward thrust is the weight of water displaced.
If you know the weight and volume, you can determine the density. You have an error with your units. The upward thrust is not 3750 Newtons. 


#3
Nov2811, 08:23 AM

P: 135

Then what is it ?



#4
Nov2811, 09:15 AM

P: 1,195

Archimedes Principle question
You are multiplying cm^3 by grams/cm^3. You do not get newtons. You get grams. A gram is not a unit of force. Newtons, however, are.
I don't know the purpose of the 10. 


#5
Nov2811, 09:25 AM

P: 1,195

kg/m^3 times m^3 times gravitational acceleration(m/sec^2) = newtons
g=9.8 m/sec^2 


#6
Nov2811, 12:51 PM

P: 247

alright , first u need to realize that the upthrust force = the wieght of the body....
ie a floating object displaces it's own wieght of the fluid in which it is floating. so, F1 = upthrust = 0.0375 * 1000 * 10 = 375 and F2 = weight of block = P*V*g =p*0.06*10 so solving this ... my answer comes out to be 0.625g/cm^3 


#7
Nov2811, 12:53 PM

P: 35

The upward force is equal to the weight of the water displaced by the body, not the displaced body.



#8
Nov2811, 12:56 PM

P: 247

so, keeping newton's 2 nd law in mind, we can come to the conclusion that : upthrust = weight of the water displaced by the block of wood and as there is no acceleration hence upthrust = actuall weight of the block Please correct me if i am wrong :) 


#9
Nov2811, 12:58 PM

P: 35

I was correcting your language, not the physics. Yes there is an upward force, but it is not a net force. I never said the upward force was the net force.



#10
Nov2811, 01:21 PM

P: 1,195

.625 gm/cm^3 is the correct result. It's density is 5/8 that of water because 3/8 of its volume remains above the surface of the water.



#11
Nov2811, 01:44 PM

P: 1,195

I had a typo in my previous message. Originally I had typed .675 but meant to type .625 gm/cm^3. It has been corrected.



#12
Nov2811, 06:21 PM

P: 135

Oh my god !! I have found the answer. Thanks everybody, thanks.



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