Fluid – a substance that can flow – normally a gas or liquid, but some solids can sometimes behave like this

• In I/GCSE Physics, fluid density is also mass per unit volume
• When an object is SUBMERGED in fluid, it feels an upward force caused by the fluid pressure – UPTHRUST
• The size of the force is equal to the weight of the fluid that has been displaced by the object. – ARCHIMEDES’ PRINCIPLE
• The mass of the fluid displaced is equal to the volume of the object x the density of the fluid
• mass = volume x density    m= Vp
• UPTHRUST is then: Weight = mass x gravity.   W=mg

Example: Why does a brick sink?

Density of water: 1000 kg m-3
If the volume of the brick is: 1.61 x 10-3

Weight of brick: 3.38kg

If we compare the weight of the brick with the upthrust when it is submerged, the resultant force will be downwards.

Weight = 3.38 x 9.81 = 33.2N downwards

Upthrust = 15.8N upwards
Resultant = 33.2 – 15.8 = 17.6 N downwards

The brick will accelerate downwards until it is at rest and balanced at the bottom

• In I/GCSE Physics, an object floats when it DISPLACES its own weight in fluid.
• When an object is at the surface of a fluid, there is NO UPTHRUST as no fluid has been displaced.
• As the object SINKS DEEPER in the fluid, it displaces a greater volume of fluid, thus INCREASING THE UPTHRUST acting upon it.
• When the upthrust and weight are BALANCED equally, the object will FLOAT.
• So if it wants to float, it has to sink and displace enough fluid to match its weight.

• In I/GCSE Physics, it is used to determine the density of a fluid. The device has a constant weight, so it will sink lower in fluids of lesser density – because a greater volume of less dense fluid must be displaced to balance the counterweight of the hydrometer. Scale markings indicate the density.

• If used with alcoholic drinks, it shows how much alcohol there is in it. The lower the density, the greater the alcohol content, as alcohol has a lower density than the water it is mixed with.

That's all~ Thanks for watching.

CLICK HERE TO LEARN MORE ABOUT OUR IGCSE/GCSE PHYSICS COURSES