Science revision for 11 - 14 year olds PHYSICS [ Forces ] [ Pressure ]

Measurement :Mass    Volume    Density

There are three fundamental units of measurement:

Mass The mass of an object tells us how heavy it is.

Mass is measured in kilograms (kg) or in grams (g)

(This means that the UNIT of mass is the kilogram)

NOTE: 1000g = 1 kg

1kg • 2 pounds

Mass is measured with a top-pan balance

Distance Distance (or length) tells us how long, thick or high something is.

Distance (or length) is measured in metres (m), centimetres (cm) or in millimetres (mm)

(this means that the UNIT of distance is the metre)

NOTE: 10mm = 1cm

100cm = 1m

1000mm = 1m

30cm = 1 foot

How to measure the mass of a sheet of paper:
Find the mass of 20 sheets and divided the answer by 20. This gave us the average mass of a single sheet

How to measure the thickness of a piece of paper:
Find the thickness of 100 sheets and divided the answer by 100. 

Volume The volume of an object is the amount of space it takes up.

The UNIT of volume is the cubic metre (m³) or cubic centimetre (cm³)

The volume of a liquid is often measured in millilitres (ml) or litres (l)

1 cubic centimetre = 1 millilitre

1000ml = 1litre

1 gallon • 4.5 litres

1 litre • 1.5 pints

The volume of a liquid is often measured using a measuring cylinder

A measuring cylinder   Volume shown = 16ml

When reading the volume of liquid in a measuring cylinder keep your eyes level with the liquid surface

NOTES: 1. Keep the measuring cylinder vertical 2. Place your eyes level with the surface of the liquid 3. Read to the lower surface of the meniscus (A meniscus is the tiny curve on the water surface where the water touches the edge of the container)

How to find the volume of a block of wood:

Measure the HEIGHT, LENGTH and WIDTH and multiply them together. Volume of block = a x b x c cm3 example: block measures 3cm x 4cm x 5cm Volume = 3 x 4 x 5 = 60cm3

How to find the volume of a small stone:
1. using a measuring cylinder

Lower the stone into a measuring cylinder containing some water and measure the volume of the displaced water.

2. Using a displacement can: Lower the object into a displacement can full of water and measure the volume of the displaced water.

1. Fill displacement can with water. 2. Place empty beaker under spout. 3. Carefully lower the object being measured into the water in the displacement can. 4. Catch the water that overflows into the beaker. 5. Pour the water in the beaker into the measuring cylinder to find its volume

The unit of time is the SECOND (s)

Time is measured using a stopwatch

The density of a substance is the mass (in grams) of one cubic centimetre of it.

Mass = density x volume Volume = mass ÷ density

If blocks of several different substances all have the same volume then the one with the largest mass is the most dense.

These cubes all have a volume of 1cm3. L

The one with the greatest mass has the highest density.

If blocks of several substances all have the same mass then the one with the highest density will have the least volume

These cubes all have a mass of 10g. L

The block with the largest volume will have the lowest density.

Finding the density of a solid with a regular shape eg a rectangular block

1. Place the block on a balance to find its mass. 2. Measure the height, width, and length and multiply them together to find he volume. 3. Divide the mass by the volume

These are the results: Mass = 120g,

Volume = height x width x length = 3cm x 4cm x 5cm

ˆ Volume = 60cm³

Density = mass/volume ˆ Density = 120 / 60 ˆ Density = 2g/cm³

1. Pour some liquid into a beaker and find the mass of the beaker and the liquid..

2. Pour the liquid into a measuring cylinder to find its volume.

3. Find the mass of the liquid by weighing the empty beaker and subtracting this from the mass of the beaker and the liquid.

(Mass of liquid = mass of liquid + container - mass of container)

4. Divide the mass of the liquid by its volume to find the density.

Finding the density of a gas eg carbon dioxide

Example Mass of boiling tube before heating = 193.5g  mass of boiling tube after heating = 193.2g ˆ loss in mass = 0.2g Volume of gas released into syringe = 78cm3 Density = mass ÷volume   ˆdensity  =  0.2÷78  = 0.0025 g/cm3

1. Find the mass of a boiling tube that contains some calcium carbonate*. 2. Set the syringe at zero and warm the boiling tube gently until the syringe is filled with gas. 3. Let the apparatus cool and then record the volume of gas released. 4. Re-weigh the boiling tube and subtract this from the original mass. The loss in mass of the calcium carbonate tells us the mass of carbon dioxide produced. 5. To find the density of carbon dioxide then divide the mass of carbon dioxide by its volume.

   (*The boiling tube can contain any chemical that produces carbon dioxide when heated.

eg To find the density of oxygen the calcium carbonate could be replaced by potassium permanganate.)

Finding the density of air

1. Set up a 1 litre flask with a stopper and tap and weigh the flask

2. Suck all of the air out of the flask and weigh it again..

3. Subtract reading 2 from reading 1. This gives you the mass of the air.

4. Find the exact volume of air removed by filling the flask with water and then pouring this water into a measuring cylinder.

4. Divide the mass of the air in the flask by its volume.

Floating and sinking

If the density of an object is MORE than the density of water (or whatever the liquid is) it will sink.

If its density is less than water it will float.

Example 1. 
the density of some oil is 0.8 g/cm³. The density of water is 1g/cm³. This means that oil will float on water.

Example 2
 Some plastic is found to have a density of 0.9 g/cm³. This means some plastic will sink in the oil but float on the water.

DENSITY: Some worked examples

A glass stopper has a mass of 55g and a volume of 11g .

a) What is the density of glass?

b) Explain how you might find the volume of the glass in the stopper

2. A bar of steel measures 5cm x 5cm x 1m. If steel has a density of 8g/cm³ what would be the mass of the bar?

Answer: To solve this problem you need to do two things:

First calculate the volume of the bar (i) and then find the mass (ii).

i) Volume of bar = 5 x 5 x 100 (Remember to convert the 1m to 100cm. You must not mix the units)

= 2500 cm³

ii) Mass of bar = density x volume

= 8 x 2500

= 20 000 grams (= 20kg)

.3.

Two liquids are poured into a beaker.

Liquid A is seen to float on to of liquid B.

A golf ball is carefully dropped into the beaker and comes to rest between the two liquids as shown in the diagram.

A 10p coin sinks to the bottom of the beaker.

Question:

a) Explain why liquid B is below liquid A

b) which has a higher density

i) the golf ball or the 10p?

Ii) The golf ball or liquid A?

Which would take up more space, 1kg of golf balls or 1kg of 10p coins? Explain.