Archimedies Principle

Up Thrust

Archimedes Principle states that when an object is partially or fully immersed in a fluid, the fluid exerts an upwards force on the object equal to the weight of the fluid displaced. This force is known as upthrust.

This is why when swimming, a person ‘feels’ like they have less weight. The person’s weight is being supported by the upthrust.

A simple experiment can demonstrate the effect of upthrust.

Archimedies Principle, figure 1

If an object is hung on a newton-meter in the air, it will extend the spring and pull down the scale needle.

Once immersed in the water the upthrust will push the object upwards reducing the pull on the newton-meter, the reading will be lower as a result.

The upthrust force is caused by the fluid that has been displaced (pushed out of the way) by the object. The fluid pushes on the object creating an upward force, called upthrust. This force is responsible for buoyancy.

The amount of upthrust depends on the weight of fluid displaced. Increasing the volume of the object will increase the weight of displaced fluid and increase the upthrust.

Archimedies Principle, figure 2

A more dense fluid creates more upthrust on the same object, because the fluid has a greater weight.

This principle is true for all fluids ie both liquids and gases.

An object does not have to be fully immersed in the fluid for there to be an upthrust force. A boat floating on the surface is held at the surface by the upthrust from the water it has displaced.


Buoyancy is the balance between the upthrust on an object in a fluid and the pull downwards due to the object’s weight (from gravity).

Positive Buoyancy (Floating)

When an object floats on the surface of a fluid, such as a ship on the sea, the weight of fluid displaced is greater than the weight of the object.

Upthrust is greater than weight

The object is pushed up to the surface and held there by the upthrust.

Negative Buoyancy (Sinking)

When an object sinks, the weight of the fluid displaced is smaller than the object’s own weight.

Weight is greater than upthrust

The upthrust slows the objects decent, but as the force of the weight downwards is greater than the upthrust the object will sink.

Neutral Buoyancy

If the weight of water displaced is equal to the weight of the object, the two forces cancel each other out and the object will hover at any given depth.

Upthrust is equal to weight

Archimedies Principle, figure 1

Density and Upthrust

The density of the fluid has an effect on the buoyancy of an object. A more dense liquid has a greater mass per volume, this means that for the same volume of liquid displaced in a more dense liquid will produce a greater upthrust.

People, ships etc are more buoyant in the sea than in freshwater. However, the density of a fluid is not only affected by the amount of dissolved salts but also by temperature. Warmer fluids are less dense.

This has some noticeable effects.

  1. Air that is heated by a hot surface, like a road on a hot summer’s day becomes less dense and so floats up through the colder more dense air around it, producing a heat haze.
  2. Commercial ships travelling between different oceans or into fresh water lakes from the sea, will have different levels of buoyancy in each. If the upthrust changes and the weight of the boat is greater than the upthrust, the ship will sink. This can happen when moving from the sea to a lake or river, (North Atlantic into the Great Lakes of Canada and the US). To prevent these disasters, ships have a loading line on their hull to show when the ship is fully loaded depending on the water it in at the time. This is called the Plimsoll Line.

Archimedies Principle, figure 2

Why might a ship sink if it sails fully laden from the sea into a freshwater river?
Your answer should include: lower / density / upthrust
Explanation: The ship might sink because the upthrust from the more dense seawater which allowed it to float in the sea, will be reduced as it enters the freshwater river. The lower density of the freshwater results in a lower upthrust. If the weight of the ship is greater than this upthrust, the ship will sink.