Upthrust

• Upthrust


  A force that determines whether an object will sink or float
• Objects in Fluids Experience Upthrust

  1. Object is submerged in a fluid

  2. Pressure of the fluid exerts a force on it from every direction

  3. Pressure increases with depth, so the force exerted on the bottom is larger than the top

  4. This causes a resultant force upwards, known as upthrust
• Upthrust


  Equal to the weight of fluid that has been displaced by the object
• An Object Floats if its Weight = Upthrust

  • If upthrust on an object is equal to the object's weight, the object floats
  • If an object's weight is more than the upthrust, the object sinks
  • Whether an object will float depends on its density
• Object less dense than the fluid

  • Displaces a volume of fluid equal to its weight before completely submerged
  • Object's weight is equal to the upthrust, so it floats
• Object denser than the fluid


  • Unable to displace enough fluid to equal its weight
  • Weight is always larger than the upthrust, so it sinks
• Submarines make use of upthrust

  1. To sink, large tanks are filled with water to increase the weight so it's more than the upthrust

  2. To rise, tanks are filled with compressed air to reduce the weight so it's less than the upthrust