Mass, Weight and Density

By: Mrs Rachel Chan – Lim

MASS AND DENSITY

Density

Density

QuestionQuestion

Have you ever wondered why a test-tube containing a certain volume of mercury feels much heavier than a test-tube containing an equal volume of water?

Density

DensityDensity

When we compare masses of equal volumes of different substances, we are actually comparing densities

SubstanceSubstance Density / kg mDensity / kg m-3-3

Mercury 13600

Water 1000

Polystyrene 16

Pinewood 500

Density

Mass

• What is Mass?

It’s the measure of the amount of matter in a body.

SI Unit: kilogram (kg)

Instruments to measure mass:

Beam balance

Electronic balance

Density

Volume

• What is Volume?

It’s the measure of the amount of space a matter takes up.

SI Unit: cubic metre (m3)

Density

Which items will float on water ???

Density

Density

•It actually means how much stuff (mass) we can fit into a space (volume).

•Example:

The more students (mass) we pack into a class (volume), the denser it would be.

The less students (mass) we pack into a class (volume), the less dense it would be.

•Density can be calculated from a ratio of mass over volume

•Equation is : Density = Mass/Volume

•SI unit: kg/m3

Density

Mercury : 13570 kg/m3

Iron : 7874 kg/m3

Glass : 2600 kg/m3

Rubber : 1100 kg/m3

Water : 997 kg/m3 1000 kg/m3

Ice : 900 kg/m3

Air : 1.225 kg/m3

Source : http://www.allmeasures.com/Formulae/static/materials/

Density of various materialsM

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Density

Density – Basic Calculation

•Let’s say a ball is 2 m3 in size. This would describe the volume.

•After weighing it on an electronic scale, it appeared that its mass is 100 kg.

•The density will then be : mass/volume

= 100 kg / 2 m3 = 50 kg/m3

•Through conversion, {(50 x 1000) g / (100)3 cm3}; it can also be described as 0.050 g/cm3

Density

Work Example

Example 1:

A block of concrete 0.4 m long, 0.3 m wide and 0.1 m high has a density of 2500 kg/m3. Calculate its mass.

Density

Solution

Solution to example 1:Given length, breadth and height of concrete block.Volume of concrete block = l x b x h

= (0.4 m)(0.3 m)(0.1 m) = 0.012 m3

From D = M / V , M = D x V = 2500 kg/m3 x 0.012 m3

= 30 kg

Density

Try Yourself!

Example 2:

A 50 cm3 glass flask is filled with oxygen and weighed. The mass is found to be 50.72 g. By means of a vacuum pump, the gas is then removed and the glass flask reweighed. The new mass is found to be 50.00 g.

Calculate the density of oxygen in (a) g/cm3 (b) kg/m3.

Density

Solution

Solution to example 2:

Mass of empty glass flash and oxygen,

m1 = 50.72 g

Mass of empty flask, m2 = 50.00 g

Volume of glass flask, V = 500 cm3

Mass of oxygen, m = m1 – m2 = 0.72 g

From D = M / V , density of oxygen

= (0.72) g / 500 cm3

= 1.44 x 10-3 g/cm3

Density

Solution

Solution to example 2:

(b) Density of oxygen in kg/m3

= [(1.44 x 10-3) x 10 -3 ]kg / [1 x 10-6] m3

= 1.44 kg/m3

Density

http://hypertextbook.com/facts/2002/AliciaNoelleJones.shtml

Facts about Milk

Density

Ice = Water??

If ice is frozen water, then why does it float on water? How is ice different from water?

Let’s use Using Density = Mass/Vol

When water is frozen into ice in a fridge,

Qn: Did the density change and why?

Qn: Did the mass change and why?

Qn: Did the volume change ?Qn: In what way has the volume changed

and how did it come about?

MASS AND WEIGHT

Density

What happens if I were to drop a book now?

Why does it act like that?

Weight

Force of gravity acting on object causes it to f allForce of gravity acting on object causes it to f all

Weight

•What is Weight?

It’s the measure of the amount of gravitational pull acting on a body.

SI Unit: Newton (N)

Instruments to measure weight:

Conversion from mass to weight:

1 kg = 10 N

So what is your weight?

Extension spring balance

Compression spring balance

Differences between Mass & Weight

On Earth

Electronic balance

Compression spring

balance

On Moon

Differences between Mass & Weight

Mass – is the same as long as the amount of matter inside is the same

Weight – it will change depending on the

pull of gravity acting on it.

e.g. (gravity)earth > (gravity)moon

(object mass)earth = (object mass)moon

(object weight)earth > (object weight)moon

Differences between Mass & Weight

The End