Density & Buoyancy of Objects: Physics Lab

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  • 0:04 What is Buoyancy?
  • 1:17 Physics Lab Steps
  • 2:50 Data Analysis
  • 4:25 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lab, you will be able to explain what buoyancy is, how to determine the size of the buoyant force, and use the principles of buoyancy to calculate the density of an object. A short quiz is available to test your knowledge after the lab.

What is Buoyancy?

Buoyancy is the ability or tendency to float in water, air or another fluid. Boats float and corks pop out of the water when submerged because of buoyancy. But why do some objects float while others sink?

You may have heard that objects float when they are less dense than water, and objects sink when they are more dense than water. And this is true. Density is how tightly packed the mass is in an object - it is the number of kilograms that each meter cubed of the material weighs. Things like foam, plastic and cork are low density, whereas metals are high density.

But to be more exact, for an object to float, the buoyant force, the force pointing upwards, has to be equal to or greater than the force of gravity on an object. Otherwise, the object will accelerate downwards and sink.

According to Archimedes' principle, the buoyant force on an object is equal to the force of gravity that would act on the displaced water. Or in other words, if two meters cubed of an object is under the water line, then the buoyant force on that object would be equal to the force of gravity that would act on two meters cubed of water. This principle is extremely useful, and today we're going to use it to calculate the density of an unknown substance.

Physics Lab Steps

For this physics lab, you will need:

  • A small block of a particular material. The material should float in the water, with a good proportion of the object submerged. (If it floats right on the top, it won't work.) A block of a fairly light wood should work well.
  • A marker that will write on the material
  • A scale
  • A ruler
  • Water in a large container (a basin, large saucepan, or a tank)

Step 1: First of all, take some measurements. Use the ruler to measure the length of each side of your block in meters, and calculate the volume of the block. (The volume of a block is length x width x height). Note down the volume of your block.

Step 2: Weigh your block, and note down its mass in kilograms.

Step 3: Float the block in the water. Once it's settled; use the marker to draw a line on it to indicate the water level at which it floats.

Step 4: Repeat the process a few times, drawing lines on other sides of the block, just to make sure your data is as accurate as possible.

Step 5: Using the ruler, measure the size of the area of the block that was submerged, in meters, and calculate the volume of the submerged part. (Again, volume = length x width x height)

If you haven't already, now it's time to pause the video and get started. Good luck!

Data Analysis

Now that you have your data, it's time to analyze it. In order to calculate the density of the block, you need two pieces of data from your experiment: the volume of the water that was displaced, and the volume of the full object. You also need to know the density of water, which is 1,000 kilograms per meter cubed.

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