Upthrust (Buoyant Force)
Have you noticed that a mug feels lighter when dipped in a bucket of water? Or that it’s hard to push an empty plastic bottle underwater?
This is because liquids exert an upward force on objects immersed in them. This upward force is called Upthrust or Buoyant Force.
Why do objects float or sink?
It is a battle between two forces:
- Gravity (Weight): Pulls the object down.
- Upthrust: Pushes the object up.
- Sinks: If Weight > Upthrust.
- Floats: If Weight = Upthrust.
- Rises: If Upthrust > Weight (like releasing a submerged plastic ball).
Archimedes’ Principle
A Greek scientist named Archimedes discovered that the upward buoyant force is equal to the weight of the liquid displaced by the object.
- If an object displaces a lot of water (like a large ship), the upthrust is huge, allowing it to float even if it is heavy!