4.5.5.1.2 - Pressure in a fluid 2

What does the pressure in a liquid depend on?

• depth

• density

How does density affect pressure?

The more dense a liquid, the more particles it has in a certain place

Meaning there are more particles that are able to collide - so the pressure is higher

As depth increases…..

…..the number of particles above that point increases

How does depth affect pressure?

As the depth increases, the number of particles above that point increases

So the weight of these particles adds up to the pressure felt at that point - so the pressure increases with depth

formula for calculating pressure due to a column of liquid:

pressure = height of the column x density of the liquid x gravitational field strength

give all the units.

pressure - pascals

height of column - in metres

density - in kg/m3

gravitational field strength - N/kg

When an object is submerged in a fluid, what exerts a force on the object from every direction?

The pressure of the fluid exerts a force on it from every direction

(when an object is either partially or fully submerged)

What is upthrust?

Upthrust refers to the resultant force upwards created when a partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surfacer (as pressure increases with depth)

What is the upthrust equal to?

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

e.g. upthrust of a pineapple in water = weight of a pineapple shaped volume of water

What determines if an object floats?

If the upthrust of an object is equal to the objects weight - then the forces balance and the object floats

When does the object sink?

If the objects weight is more than the upthrust - then the object sinks

So what influences an objects weight, and therefore whether an object with float or sink?

The objects density

Describe how density affects whether an object floats or sinks.

E.g. an object that is less dense than the fluid it’s placed in weighs less than the equivalent volume of the fluid

Meaning it displaces a volume of fluid that is equal or less to its weight before it is fully submerged

E.g. an object that is denser than the fluid it is placed in , is unable to displace enough fluid equal to its weight so it sinks.

How do submarines sink?

To sink, submarines have large tanks that are filled with water to increase the weight of the submarine so that is is more than the upthrust

How do submarines float back up?

To rise back to the surface - the tanks are filled with compressed air to reduce the weight , so that is is less than the upthrust