Upthrust and Atmospheric Pressure

• When an object is submerged in a fluid the pressure of the fluid exerts a force on it from every direction.

  Pressure increases with depth so the force exerted in the bottom of the object is larger than the force acting on the top of the object.

  This causes a resultant force upwards known as upthrust
• This upthrust is equal to the weight of the fluid that has been displaced by the object
• If the upthrust on an object is equal to the object‘s weight, then the forces balance and the object floats.

  If an object’s weight is more than the upthrust, the object sinks.

  Whether or not an object will float depends on its density
• An object that is less dense than the fluid it is placed in weighs less than the equivalent volume of fluid. This means it displaces a volume of fluid that is equal to its weight before it is completely submerged.

  At this point, the object‘s weight is equal to the upthrust so the object floats.
• An object that is denser than the fluid it is placed in is unable to displace enough fluid to equal its weight. This means that its weight is always larger than the upthrust so it sinks.
• Submarines make use of upthrust. To sink, large tanks are filled with water to increase the weight of the submarine so that it is more than the upthrust. To rise to the surface, the tanks are filled with compressed air to reduce the weight so that it’s less than the upthrust
• The atmosphere is a layer of air that surrounds Earth. It is thin compared to the size of the Earth
• Atmospheric pressure is created on a surface by air molecules colliding with the surface
• As the altitude increases, atmospheric pressure decreases.

  This is because as the altitude increases, the atmosphere gets less dense so there are fewer air molecules that are able to collide with the surface.

  There are also fewer air molecules above a surface as the height increases. This means that the weight of the air above it decreases with altitude.