Archimedes’ Principle

Archimedes’ Principle is the cornerstone of understanding buoyancy. It states that the upward buoyant force exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces. But what does that really mean? Let’s break it down.

Imagine submerging a sealed, empty bottle in water. The bottle pushes water out of the way — it displaces it. Archimedes’ Principle tells us that the water is pushing back against the bottle with a force equivalent to the weight of the water displaced by the bottle. This is why the bottle feels lighter when submerged; the water’s push, or the buoyant force, is helping to support it.

This principle doesn’t just apply to water; it applies to all fluids, including gases like air. It’s why balloons rise and why life jackets keep you afloat. The principle is also indifferent to the state of the fluid – whether the fluid is in motion or at rest, the buoyant force remains consistent.

Now, let’s express this principle mathematically:

\[ F_b = \rho_{fluid} \times V_{displaced} \times g \]


  • \( F_b \) is the buoyant force,
  • \( \rho_{fluid} \) is the density of the fluid,
  • \( V_{displaced} \) is the volume of the fluid displaced, and
  • \( g \) is the acceleration due to gravity.

This formula is a powerful tool in predicting the behavior of objects in a fluid. It’s the formula you’ll use to calculate whether an object will float, suspend, or sink. It’s also a formula that can lead to some profound insights into the nature of fluids and their interactions with solid objects.

As we progress, we will apply Archimedes’ Principle to various scenarios, deepening our understanding and preparing us to tackle questions with confidence.

Calculating Buoyant Forces

When an object is placed in a fluid, it experiences a buoyant force. To calculate this force, we use Archimedes’ Principle, which provides a straightforward method. Here’s how you can compute the buoyant force acting on an object:

  1. Determine the Volume of the Displaced Fluid:
    • First, you need to find the volume of the fluid that the object displaces. For a submerged object, this is simply the volume of the object itself. For floating objects, it’s the volume of the object that is underwater.
  2. Calculate the Buoyant Force:
    • Multiply the volume of the displaced fluid by the density of the fluid* and the acceleration due to gravity (\( g \)) to get the buoyant force: \[ F_b = \rho_{fluid} \times V_{displaced} \times g \]

*One of the biggest mistakes students make is accidentally using an object’s density rather than the fluid density to calculate the buoyant force. Remember it is the fluid that causes the buoyant force so we need to use the fluid’s density too!

Example Calculation:

Let’s consider a block of a substance with a volume of \( 2.0 \) cubic meters (m³) submerged in fresh water. The density of fresh water is approximately \( 1000 \) kilograms per cubic meter (kg/m³). We will use \( g = 10 \) meters per second squared (m/s²) for simplicity. What is the magnitude of the buoyant force experienced by the block.

Calculating The Buoyant Force

\[ F_b = \rho_{water} \times V_{block} \times g \] \[ F_b = 1000 \, \text{kg/m}^3 \times 2.0 \, \text{m}^3 \times 10 \, \text{m/s}^2 \] \[ F_b = 20000 \, \text{kg} \cdot \text{m/s}^2 \] \[ F_b = 20000 \, \text{N} \] (since \( 1 \) kg·m/s² is equal to \( 1 \) Newton (N))

This means the block will experience an upward buoyant force of 20,000 N.