How Much Lead Is Needed to Sink a Floating Log?

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In summary, to sink a 2.04 kg piece of wood with a mass of lead hung from it by a string, 21.42N is needed.
  • #1
moniquert
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Homework Statement



A 2.04 kg piece of wood floats on water of density 1006.2 kg/m3. If the density of wood is 486.0 kg/m3 and the density of lead is 11300 kg/m3, what minimum mass of lead, hung from it by a string, will cause it to sink?

Homework Equations


Bouyant force= (density of water)(Volume displaced)(gravity 9.81)
Volume= (Pie)(r)squared(height)
Density= mass/volume


The Attempt at a Solution



I found the weight of the log by doing (9.81)(2.04)=20.0124N. Then i found volume of the log (2.04)/486=0.004197531 metres cubed. Then i found bouyant force upwards (1006.2)(.004197531)(9.81)= 41.43308N. Then I subtracted them to get how much weight is required for it just to stay a float 21.42N. I don't Know what to do after that help please.
 
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  • #2
Well, you have mass of lead. Find Fb which should be equal to weight of lead. Also, judging from the volume in 2, you didn't mention that is was a cylinder in 1.
 
  • #3
I don't have the mass of lead only the density. and how do you know its a cylinder? and if it is a cylinder i still can't find the volume that way because i don't have the radius or height.
 
  • #4
2. Homework Equations
Bouyant force= (density of water)(Volume displaced)(gravity 9.81)
Volume= (Pie)(r)squared(height)
Density= mass/volume

Isn't that volume of cylinder?
 
  • #5
I know that's a volume of a cylinder but i don't have the radius or height to use that equation so i just used the density=mass/volume
 
  • #6
Well, you are right up to the last part of 21.398N. You also need to account for the Fb of the lead underwater.
 
  • #7
To find the fb of lead would i just do density of lead times volume of the log? I don't know how to solve for the Fb of lea what numbers to use. (11300)(0.004197531)(9.81)=465.3N
 
  • #9
moniquert said:
To find the fb of lead would i just do density of lead times volume of the log? I don't know how to solve for the Fb of lea what numbers to use. (11300)(0.004197531)(9.81)=465.3N

The buoyant force of the lead is the weight of the water that it displaces.
 
  • #10
moniquert said:

The Attempt at a Solution



I found the weight of the log by doing (9.81)(2.04)=20.0124N. Then i found volume of the log (2.04)/486=0.004197531 metres cubed. Then i found bouyant force upwards (1006.2)(.004197531)(9.81)= 41.43308N. Then I subtracted them to get how much weight is required for it just to stay a float 21.42N. I don't Know what to do after that help please.
You're on the right track, but it's a bit trickier than that.

It's always good idea to draw a diagram and label all the forces involved. The diagram would show the submerged wood and a piece of lead (also submerged) hanging down from it by a string.

What forces act on the wood? The buoyant force acting up; the weight and string tension acting down. You've calculated what the string tension must be.

What forces act on the lead? String tension and buoyant force acting up; the weight acting down. Use this information to calculate the volume and thus the mass of lead needed.
 
  • #11
Forget the string and think of the lead and wood just below the surface. The total buoyant force on the lead and wood must be equal to the weight of the lead and wood. The buoyant force is the weight of the displaced water; the volume of the displaced water is the same as the volume of the lead and wood. Set this up and see what you have.

Doc Al said:
You're on the right track, but it's a bit trickier than that.

It's always good idea to draw a diagram and label all the forces involved. The diagram would show the submerged wood and a piece of lead (also submerged) hanging down from it by a string.

What forces act on the wood? The buoyant force acting up; the weight and string tension acting down. You've calculated what the string tension must be.

What forces act on the lead? String tension and buoyant force acting up; the weight acting down. Use this information to calculate the volume and thus the mass of lead needed.
 
  • #12
That's a perfectly fine way to go as well. :wink:
 

Related to How Much Lead Is Needed to Sink a Floating Log?

What is the definition of density?

Density is defined as the measure of mass per unit volume of a substance. It is commonly measured in units of grams per cubic centimeter (g/cm3) or kilograms per cubic meter (kg/m3).

How is density calculated?

Density is calculated by dividing the mass of a substance by its volume. The equation for density is: Density = mass / volume.

What is the difference between a liquid and a solid?

A liquid has a definite volume, but takes the shape of its container. In contrast, a solid has both a definite volume and shape.

How does temperature affect the density of a liquid?

In general, as temperature increases, the density of a liquid decreases. This is because as temperature increases, the molecules in a liquid have more energy and move further apart, making the substance less dense.

What is the density of water?

The density of water is 1 g/cm3 at 4 degrees Celsius. This means that 1 cubic centimeter of water has a mass of 1 gram. However, the density of water can vary slightly with temperature and pressure changes.

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