Knowt
Upthrust and Atmospheric Pressure
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
Upthrust and Atmospheric Pressure
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
