Floating (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

Density and floating

• Whether an object floats or sinks depends on the relative densities of the object and the fluid it is submerged in
  • If the object is denser than the fluid, it will sink
  • If the object is less dense than the fluid, it will float

• Upthrust is a force that pushes upwards on an object submerged in a fluid 
• Upthrust always acts in the opposite direction to the object's weight force
  • This is how objects float

• The size of the upthrust force depends on:
  • the density of the fluid
  • the volume of fluid that is displaced (which is equal to the volume of the object)
  • The denser the liquid, the greater the force of upthrust exerted on the object 

Upthrust and weight forces on a boat

An object will float if the force of upthrust is equal to the force of weight

Factors affecting floating & sinking

• The factors affecting floating and sinking are:
  • upthrust
  • density
• If the force of upthrust on an object is:
  • equal to the object’s weight, then the object will float
  • less than the object’s weight, then the object will sink

• If the density of the object is:
  • greater than the density of the fluid, then the object will sink
  • less than the density of the fluid, then the object will float

• If the density of the object is greater than the density of the fluid, the object can never displace enough fluid to create an upthrust that will equal its weight (and therefore it sinks)

Objects which are less dense than water will float in water, and those that are more dense will sink

• A polystyrene block will float in water
  • This is because polystyrene has a density of 0.05 g/cm³ which is much less than the density of water (1.0 g/cm³)

• A wooden block will be partially submerged but will still float
  • This is because the density of a wooden block (0.9 g/cm³) is slightly less than the density of water

• An iron block will sink
  • This is because iron has a density (7.9 g/cm³) that is much higher than water