Physics Additional Notes AS level

Upthrust  

- When an object is partly or completely submerged in a liquid, the pressure at the bottom surface is greater than the pressure at the top surface.  

- This pressure difference produces a resultant upward force called upthrust (or buoyant force).  

P = D x g x h

F = P x A

Force at the top surface = D x g x A x h1

Force at the bottom surface = D x g x A x h2

Resultant force = F(bot) - F(top)

Resultant force = D x g x A (h2 - h1)

But h2 - h1 = height of the object

A x (h2-h1) = Volume of the object

Therefore, Upthrust = D x g x V

where: D = density of liquid, g = acceleration due to gravity, V = volume of object

Archimedes’ Principle 

- If a container is filled with liquid and an object is immersed in it, the upthrust acting on the object equals the weight of the displaced liquid.  

- Mathematically:  

  Upthrust = Weight of displaced liquid 

Key Points  

- Upthrust explains why objects float or sink.  

- If the upthrust is greater than the object’s weight, the object floats.  

- If the upthrust is less than the object’s weight, the object sinks.  

- This principle is used in designing ships, submarines, and hydrometers.  

Concept of Apparent Weight

- When an object is submerged in liquid, its weight appears to be less than the real weight.

Upthrust = Real weight - Apparent weight = Loss in weight of an object.