Objects in fluids experience upthrust

• 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 on 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

• The upthrust is equal to the weight of fluid that has been displaced by the object

An object floats if its weight=upthrust

• 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

Atmospheric pressure decreases with height

• 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, which contributes to atmospheric pressure, decreases with altitude