Science 8: Mix and Flow of Matter Topic 6 - Buoyancy

What is Buoyancy? Buoyancy is the tendancy for materials to rise or float in a fluid (usually water). Buoyant Force - the upward force exerted on objects that are placed in a fluid. Like all forces it’s measured in newtons (N) and its symbol is FB

The Buoyant Force Pushes up on an object in the fluid. Therefore, it is called the anti-gravity force. The FB comes from the particles of the fluid pushing against the weight of the object. If FB of the object is greater than the Fg of the object it will float. If it is less, it will sink. Neutral Buoyancy – If the FB acting on the object is equal to the object’s Fg then it will neither float nor sink, but exist half way down the fluid.

What does the amount of FB of an object depend on? The amount FB depends on the WEIGHT OF THE AMOUNT OF WATER DISPLACED BY THE OBJECT. This is the Archimedes Principle discovered ~250 BC. What does this mean? If the average density of the object is more than water’s >1g/ml it will sink. If it’s <1g/ml, it will float.

What’s Average Density? Average Density - total mass of all materials enclosed in an object, divided by its total volume. Eg. Huge metal oil tankers can float on water because they are mostly hollow and full of very light air.

Why is Average Density Useful? Because it enables dense materials to be used in floating devices such as ships, boats, etc. Imagine an oil tanker full of millions of litres of oil being protected in a wooden or fibreglass container. It’s also useful with submarines. Simply by adding or removing water the sub becomes more dense or less dense than the ocean, causing it to sink or rise.

What is a Cartesian Diver? A simple device that shows buoyancy and pressure. The eye-dropper is buoyant in the water; however when the bottle is pressurized (squeezed) the pressure causes water to enter the dropper making it more dense and less buoyant, causing it to sink.

What is a Hydrometer? an instrument used to directly measure the density of a liquid. The higher the indicator floats in the liquid, the denser the liquid. Used in the food industry to see how much solute is in soft drinks, wine, etc. Used in the food industry to see how much solute (eg. Salt, sugar) is in soft drinks, wine, etc. The more solute a liquid has, the denser it is, the denser it is, the more FB It exerts.