Density Density is a comparison of how much

Density • Density is a comparison of how much matter there is in a certain amount of space. • Which square is more dense?

Calculating Density • Density = mass volume – solids: d = grams/cubic centimeters (g/cm 3) – liquids: d = grams/milliliters (g/ml) ALWAYS REMEMBER THE UNITS! To get the mass just weigh the object

Volume Length Height Width • for regular shapes – volume = length x width x height e. g. = 10 cm 3

• for irregular shapes – submerge object in a liquid – measure the volume of liquid that is displaced in ml – e. g. key displaced 3 ml of liquid therefore its volume is 3 ml • for liquids – measure directly in ml – e. g. 50 ml

Buoyancy • BUOYANCY is the tendency of an object or substance to float Gravity – downward force of gravity pulling on the object Buoyant Force – upward force of fluid pushing on the object

• if gravity > buoyant force → object sinks • neutral buoyancy gravity = buoyant force object stays suspended • if gravity < buoyant force → object rises or floats

How are Density and Buoyancy Related?

How can dense things float? • the density of steel is 9. 0 g/cm 3 • the density of water is 1. 0 g/ml and the density of sea water is 1. 03 g/ml • how can a ship made of steel float? Archimedes discovered that: A ship will float when the weight of the water it displaces equals the weight of the ship. AND Anything will float if it is shaped to displace its own weight of water before it reaches the point where it will submerge.

Average Density • average density – the total mass of ALL the substances that make up an object divided by the total volume of the object • humans, fish, ships, submarines, etc can float because of average density – they have lots of air or water spaces inside them

Archimedes Principle • Buoyant Force = weight in air – weight in fluid e. g. 7 lbs – 4 lbs = buoyant force of 3 lbs and the weight of the water displaced by the object is also 3 lbs

Don’t forget air is also a fluid… • so the principles that apply to having something float in water also apply to having something float in air!

Hydrometer • uses buoyancy to measure density directly • calibrated with markings to show density in g/ml, • a denser liquid holds the hydrometer up higher • a less dense liquid allows the hydrometer to sink more