Floating (CIE IGCSE Physics)

Upthrust

• Upthrust is a force that pushes upwards on an object submerged in a fluid i.e. liquids and gases
• It is always in the opposite direction to the object's weight
  • This is why boats, and objects that are less dense than water, float

• The size of the upthrust depends on the density of the fluid as well as the volume of fluid that is displaced (which is equal to the volume of the object)
  • The denser the liquid, the greater the upthrust it will exert on an object

Upthrust is in the opposite direction to the weight of the boat and the fisherman

Factors Affecting Floating & Sinking

• Whether an object sinks or floats depends on the upthrust:
  • If the upthrust on an object is equal to (or greater than) the object’s weight, then the object will float
  • If the upthrust is smaller than the weight then the object will sink

• The outcome also depends on the object's density:
  • If it has a density less than the density of the fluid it is immersed in, the object will float
  • If it has a density more than the density of the fluid it is immersed in, the object will sink

• This is because 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 hold its weight up (and therefore sinks)

Objects which are less dense than water will float and which 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

EXTENDED

• A liquid with a lower density will float on a liquid with a higher density if the liquids do not mix

Lower density liquids float on higher density liquids as long as the liquids do not mix

Step 1: List the known quantities

• • Liquid A = 0.76 g/cm³ 
  • Liquid B = 0.93 g/cm³

Step 2: Determine which liquid has the lowest density

• • The liquid with the lowest density will float on top of the liquid with the higher density
  • 0.76 is less than 0.93
  • Therefore, liquid A has the lowest density

Step 3: State your answer

• •    Liquid A will float on top of liquid B