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How Do You Calculate Negative Buoyancy?

Published in Physics of Buoyancy 3 mins read

You calculate negative buoyancy by determining if an object's weight is greater than the buoyant force acting upon it, which is equivalent to the weight of the fluid the object displaces. If the object's weight exceeds the buoyant force, it will sink.

Here's a breakdown:

  • Understanding Buoyancy: Buoyancy is the upward force exerted by a fluid that opposes the weight of an immersed object. This force is what makes objects float or appear lighter in water.

  • Negative Buoyancy Defined: Negative buoyancy occurs when the weight of an object is greater than the buoyant force acting on it. This results in the object sinking.

  • The Calculation: The core principle hinges on Archimedes' principle:

    • Buoyant Force (BF) = Weight of Fluid Displaced

    To determine negative buoyancy, you need to compare two values:

    1. Weight of the Object (Wo): This is the force due to gravity acting on the object's mass (Wo = mass * gravity).
    2. Buoyant Force (BF): This is the weight of the fluid that the object displaces when submerged (BF = volume of displaced fluid density of fluid gravity).

  • The Rule:

    • If Wo > BF, the object experiences negative buoyancy and sinks.
    • If Wo < BF, the object experiences positive buoyancy and floats.
    • If Wo = BF, the object experiences neutral buoyancy and remains suspended.

  • Factors Influencing Negative Buoyancy:

    • Density of the Object: A denser object (more mass per unit volume) is more likely to sink.
    • Density of the Fluid: An object is more likely to float in a denser fluid (like saltwater compared to freshwater).
    • Volume of the Object: A larger volume displaces more fluid, increasing the buoyant force.

  • Example:

    Imagine a rock. Its weight (Wo) is 5 Newtons. When submerged in water, it displaces a certain volume of water. The weight of that displaced water (BF) is only 3 Newtons. Since 5 N > 3 N, the rock exhibits negative buoyancy and will sink.

  • Formula Summary:

    Negative Buoyancy Exists When:

    • Weight of Object > (Volume of Displaced Fluid Density of Fluid Gravity)
    • Wo > (V ρ g)

In essence, calculating negative buoyancy means confirming that the object is heavier than the amount of fluid it pushes aside. This difference in forces determines whether an object sinks.

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