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Density. Density is a comparison of how much matter there is in a certain amount of space. Which square is more dense?. Density = mass volume solids: d = grams/cubic centimeters (g/cm3) liquids: d = grams/milliliters (g/ml). Calculating Density. ALWAYS REMEMBER THE UNITS!.

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density
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/cm3)

liquids: d = grams/milliliters (g/ml)

Calculating Density

ALWAYS REMEMBER THE UNITS!

To get the mass just weigh the object

volume
Volume
  • for regular shapes
    • volume = length x width x height

e.g. = 10 cm3

Length

Height

Width

slide4
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
  • 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

slide6
if gravity > buoyant force → object sinks
  • neutral buoyancygravity = buoyant force object stays suspended
  • if gravity < buoyant force → object rises or floats
how can dense things float
How can dense things float?
  • the density of steel is 9.0 g/cm3
  • 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
  • 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
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
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
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