How to Calculate Tension Underwater

Use the beaker's measurement marks to measure water level.
Use the beaker's measurement marks to measure water level. (Image: Beaker With Blue Liquid image by explicitly from

It is possible to find an object's density by dividing the object's mass by its volume. According to the Archimedes principle, an object completely immersed in water or any other liquid experiences a buoyant force equal to the amount of liquid that it displaces. The tension of a string tied to the object under water can be measured with a few simple calculations.

Things You'll Need

  • String
  • Marble
  • Beaker
  • Water
  • Hook weighing scale
  • Ruler

Fill water to the halfway point of the beaker. Tie the string around the marble.

Weigh the marble with the weighing scale by tying the string around the hook and leaving the marble suspended in air. Log the measurement.

Lower the marble into the beaker of water. Log the measurement for the rise in water when the marble is lowered into the water. Tie the string around the hook of the weighing scale and measure the weight of the rock suspended in water.

Calculate the volume of the marble. Measure the radius of the marble with your ruler and use the value in the following formula:

Volume = 4/3_π_R^3

where R is the radius of the marble.

Calculate the tension of the string with the following formula:

T = mg - Fb

where mg is the rise in water logged in Step 3 and Fb = (density of water)*(volume of marble). Use the value of 1 kg/liter for the density of water and the value calculated in Step 4 for the volume of the marble.

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