Archimedes Principle: Calculating Relative Density for a Block of Oak and Lead

In summary, the conversation discusses the calculation of the relative density of a block of oak. By using the formula F=mxg, the weight of the oak in air and water is determined to be 90 N and -30 N respectively. Through the equation F(archimedes)= density x Volume x g, the volume of the oak is calculated to be 0.00611 m3. Finally, the relative density is found to be 1.499, which is incorrect. After correcting for a sign error, the correct answer of 0.75 is obtained. The conversation also highlights the concept of negative net force and its implications on the behavior of an object in water.
  • #1
Nanu Nana
95
5
. Homework Statement
A block oak weighs 90 N in air. A piece of lead weighs 130 N in water. The two weigh together 100N in water. Calculate the relative density of the wood.

Homework Equations


F=mxg
F(archimedes)= density x Volume x g

The Attempt at a Solution


F (g)oak = 90 N
F= mxg
m= 90N/9.81 m/s2
m= 9.17 gram
Oak in water = 130-100 N =30 N
Farchimedes oak = 90N-30N =60N
60N =1000kg/m3 x V x 9.81 m/s2
V=60N/9810
V=0.00611
Now that I have calculated V I can find density
rho = 9.17kg/ 0.00611 m3
density = 1499
Relative density is 1499 /1000 kg/m3 = 1.499
But the correct answer is 0.75
 
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  • #2
Check your signs. In water, oak+lead=100N, while lead =130N.
 
  • #3
How to do find N Force of oak in water ??
 
  • #4
Nanu Nana said:
How to do find N Force of oak in water ??
Your method was ok but you made a mistake with the signs. Manipulate the two equations I wrote above.
 
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  • #5
100+130?
 
  • #6
Nanu Nana said:
100+130?
Please, don't make wild guesses.
If Ow+Lw=100 and Lw=130, what is Ow?
 
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  • #7
Sorry /D its -30 so 90-(-30) = 120
 
  • #8
Nanu Nana said:
Sorry /D its -30 so 90-(-30) = 120
Right. Does this get you to the right answer?
(How do you interpret the weight of -30N?)
 
  • #9
haruspex said:
Right. Does this get you to the right answer?
(How do you interpret the weight of -30N?)
Yes I just calculated it. Negative force means opposite direction . But i don't know in this context
 
  • #10
Nanu Nana said:
Yes I just calculated it. Negative force means opposite direction . But i don't know in this context
If the wood were immersed in the water without being bound to the lead, what would happen?
 
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  • #11
It floats ofcourse
 
  • #12
Nanu Nana said:
It floats ofcourse
Right, hence the negative net force.
 
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  • #13
haruspex said:
Right, hence the negative net force.
Oh I see . thank you very much :)
 

1. What is Archimedes Principle?

Archimedes Principle is a scientific law that states that the buoyant force acting on an object submerged in a fluid is equal to the weight of the fluid that the object displaces.

2. How is Archimedes Principle used to calculate relative density?

Archimedes Principle can be used to calculate relative density by comparing the weight of an object in air to its weight when submerged in a fluid. The difference in weight is equal to the weight of the fluid that is displaced by the object, which can then be used to calculate relative density.

3. What is relative density?

Relative density, also known as specific gravity, is a measure of the density of a substance compared to the density of a reference substance. In this experiment, the relative density of the block of oak and lead is calculated by comparing their densities to the density of water.

4. Why is it important to calculate relative density?

Calculating relative density can provide valuable information about the physical properties of a substance, such as its purity or composition. It can also be used to determine the suitability of a substance for certain applications, such as in construction or manufacturing.

5. What factors can affect the accuracy of the relative density calculation?

The accuracy of the relative density calculation can be affected by factors such as the precision of the measuring instruments used, the temperature and pressure of the fluid, and any impurities or air bubbles present in the fluid. It is important to carefully control these factors in order to obtain accurate results.

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