lesson

The title of the laboratory work: Identify floating condition

Purpose of the work: 

  • Identify with the help of experiment the condition needed for bodies to float in liquid;
  • Compare the force of gravity to the Archimedes’ force acting on the body that floats on the surface of liquid.

Expected results:

  • The condition for a body to float in liquid is that the Archimedes’ force acting on the body must be greater than the force of gravity.
  • When a body floates on the surface of the liquid, the Archimedes’ force acting on it is greater than the force of gravity.

Teacher’s Guide:

  • Divide the students to 3 students in each group.
  • Before starting laboratory work, read the safety rules: 
  • To download the worksheet, please follow this link:

Theoretical part

Archimedes’ force is the buoyant force that acts on the body in the liquid or gas and pushes it upwards.

The formula of the Archimedes force is

FA = ρ V g

where ρ  is the density of the liquid, V is the volume of the body immersed into the liquid, g is the free fall acceleration. 

The force of gravity is the force that acts on bodies because of Earth’s gravitational attraction and is identified by the formula:

F = m g

where m is the mass of the body.

Sources:

  1. Выталкивающая сила. (2019). Физика. Учебник для 7 класса общеобразовательной школы. 
  2. Явление тяготения и сила тяжести. Вес. (2019). Физика. Учебник для 7 класса общеобразовательной школы.

Experimental part

Experiment 1. Compare Archimedes’ force with the weight of the liquid displaced by the body. 

In this lab we need a beaker, a test tube, dynamometer, a napkin, and a container for water.

Procedure:

Step 1.

Measure the volume of the test tube using a beaker. Pour water into the beaker, measure the level of water, and then put in the test tube and measure the level of water again. The difference of levels is the volume of the test tube. 

Step 2. Calculate the Archimedes’ force for this test tube according to the formula:

FA = ρ V g

where ρ  is the density of the liquid, V is the volume of the body immersed into the liquid, g is the free fall acceleration. The density of water is ρ = 1000 kg/m3, g = 9.8 N/kg. Write down the data to the table.

Step 3.

Put the tube into water and make sure that the tube is floating. Take it out from the water, wipe with the napkin and measure the weight of the tube using a dynamometer. Record the data to the table.

Step 4.

Fill in the tube with water and put it into the container. Make sure the tube sinks, then take the tube out, wipe it off and then measure the weight of the tube using a dynamometer. Record the data to the table. 

Step 5.

Pour out some water of the tube, such that the tube floats inside the water. Put into water, make sure it floats inside the water, then take it out, wipe it off and then measure the weight of it using dynamometer. Record the data into the table. 

Step 6. Compare the Archimedes’ force to the force of gravity of the tube for these 3 cases. Draw a conclusion about that result. 

Experiment 2. Compare Archimedes’ force to the force of gravity of the body that floats at the surface of the liquid.  

Procedure:

Step 1. Measure the weight of the body and take a note of the record. 

Step 2. Measure the volume of the body using a beaker and take a note of the record. 

Step 3. Calculate the Archimedes’ force according to the formula:

FA = ρ V g

Step 4. Compare the Archimedes’ force to the force of gravity of the body. 

Step 5. Draw a picture of the body floating at the surface of the liquid and draw the forces that act on the body. Derive conclusions.  

Research part

Volume,
V, ml
Archimedes’ force, 
FА, N
Weight,
P, N
Weight with error,
P±ΔF, N
P to FА
fraction
Behavior of the tube
in water
1      
2      
3      

When deriving conclusions, answer the following questions:

  • What is the floating condition for the body?
  • When a body floats at the surface of the liquid, what forces act on the body? Which of them is bigger? 

Conclusion

The floating condition for the body in the liquid is that the Archimedes’ force acting on the body must be greater than the force of gravity. When a body floats at the surface of the liquid, the Achimedes’ force acting on the body is greater than the force of gravity.