How to Calculate Buoyancy Force

Buoyancy makes objects float.

When an object is put in a fluid, that fluid exerts pressure on all sides of the object. Pressure from below exerts a force on the object that "buoys" it, or pushes it upward. If the object is less dense than the fluid, it will float. Even if the object is more dense than water, buoyancy pushes up on the object and changes its perceived weight. Archimedes discovered that the force with which the fluid pushes a submerged object up is equal to the weight of the water that the object displaces. Knowing the mass of the object lets you calculate the buoyancy force.

Things You'll Need

• Spring scale
• Beaker of water

Instructions

1. Measure the mass of your object in grams with the hanging spring scale. This is the true mass (m1) of the object.

2. With the object still hanging from the spring scale, lower the object into the water until it floats or is completely submerged. Measure the mass (m2) of the object in water.

3. Subtract the mass of the object in water (m2) from the true mass of the object (m1). This is equivalent to the mass of the water in grams displaced by the object (m3). For example, (m1) - (m2) = (m3).

4. Divide your answer by 1,000 to convert grams to kilograms. For example, (m3 in grams) x 1 kg / 1,000 g = (m3 in kilograms).

5. Multiply your answer by the acceleration of a falling object due to gravity, which is 9.81 meters per second per second. This determines the force in newtons, the standard metric unit of force and weight. This is the force of buoyancy on your object.

References

• Georgia State University; Pressure
• Davidson College; The Buoyant Force