Lesson - 1.3.4

• 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

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)