GRADE 8 SCIENCE 2011

DENSITY & BUOYANCY

DENSITY AND MATTER

All matter exists in one of three states:

•Solid – has a definite shape and volume (e.g. Sugar)• •Liquid – has a definite volume but no definite shape (e.g. Water)• •Gas – no definite shape nor a definite volume (e.g. Oxygen)

DENSITY OF SOLIDS, LIQUIDS & GASES

• Does water as a liquid, water a solid, or water as a gas have the highest density? Explain.

MATTER

• All matter is made up of very small particles.

• All particles in a pure substance are the same.

• There is space between the particles.

• The particles are always moving.

• The particles in a substance are attracted to each other. The strength of the attraction depends upon the type of particle.

WHAT IS DENSITY?

• Density is a comparison of how much matter there is in a certain amount of space.

MASSMASS

• Mass is a measure of the amount of substance • Mass cannot be changed by the

location, shape and speed of the body.• Large masses are measured in tonnes

while small masses are measured in grams.

WEIGHT IS . . .WEIGHT IS . . .

The gravitational force acting on an object

Can be altered or affect due to location

Weight is measured in newtons (N)

DIFFERENCES BETWEEN WEIGHT DIFFERENCES BETWEEN WEIGHT AND MASSAND MASS

Weight Mass

pull of gravity on the body/objectmeasured in newtons changes from place to place

measurement of the amount of matter in the body/objectmeasured in kilograms/grams, etc is constant regardless of place or location

DENSITY

• Density can be described as the “crowdedness” of the particles that make up matter. • When you describe something as being “heavy”

or “light” you are referring to that something’s density. • Each substance has its own, unique density. • The closer the particles are together in a

substance, the higher its density is.

I love Density

DENSITY

I LOVE DENSITY

Density =

Density =

DENSITY FORMULA

Density = Mass Volume

 

Units g/ml, g/cm3,

ALWAYS REMEMBER

UNITS

THE ARCHIMEDES PRINCIPLE

Ancient Greek Mathematician (aka the streaking Greek)

proved that an object immersed in water always displaced a volume of water equal to its own volume

Was tasked with proving that a gold crown for King Heron II was in fact made of solid gold. The king thought that a sneaky jeweler had replaced some of the gold with a less valuable metal when he made it.

Also famous for sprinting nude from his house when coming to this realization.

THE ARCHIMEDES PRINCIPLE

THE ARCHIMEDES PRINCIPLE

WHICH ONE IS MORE DENSE?

• Demonstration: People in a square

• How about this: Which square is more dense?

WHICH ONE IS MORE DENSE?

• Now which one is more dense?

DENSITY OF SOLIDS, LIQUIDS & GASES (CONT.)

• Solid object can move easily through liquids and gases. • Example: Dolphins can leap through the air and then dive

back underwater so smoothly that the activity appears almost effortless.

According to the particle theory, the particles in liquid and gases get out of the way of for solids.

Why do solid particles tend to hold together while fluid particles tend to move apart?

DENSITY OF SOLIDS, LIQUIDS & GASES (CONT.)

• Answer: When a solid object move through a fluid, it pushes apart and move between them. Particles in a solid cannot be pushed apart. • Attractive forces among the particles of a solid

are stronger than those between fluid particles. Therefore, the particles in a solid cannot be pushed apart.