Transcript Density - SJAM Science with Ms. Morrison / SJAM Science

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)
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 cm3
• 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/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 – 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