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**on the bottom of the object is**larger**than the force acting on the top of the objectThis causes a

**resultant**force**upwards**, known as**upthrust**The upthrust is

**equal**to the weight of 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 floatsIf 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 submergedAt this point, the object’s weight is

**equal**to the upthrust, so the object floatsAn 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

The atmosphere is a

**layer of air**that surrounds Earth. It is**thin**compared to the size of the EarthAtmospheric pressure is created on a surface by

**air molecules collidin**g with the surfaceAs 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 surfaceThere 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

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 objectThis causes a

**resultant**force**upwards**, known as**upthrust**The upthrust is

**equal**to the weight of 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 floatsIf 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 submergedAt this point, the object’s weight is

**equal**to the upthrust, so the object floatsAn 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

The atmosphere is a

**layer of air**that surrounds Earth. It is**thin**compared to the size of the EarthAtmospheric pressure is created on a surface by

**air molecules collidin**g with the surfaceAs 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 surfaceThere 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