Unit 5 Matter and Forces

TOPIC 5 MATTER AND FORCES

Matter is everything around us. Matter is made out of tiny particles called atoms. Some atoms join together to make groups known as molecules.Apart from its colour, odour and taste, matter has other properties and can be found in three different states.

Volume describes how much space matter occupies. A car has a larger volume than a bicycle. We measure volume in millilitres (ml).
1 000 millilitres = 1 litre

Density is the amount of matter in a volume. We measure density in kilograms per litre (kg/l).
density = mass/volume
Density explains why some objects float in water while others sink.

Hardness is the scratch-resistance of a solid. Diamonds are the hardest natural solids.

Solubility is the ability of a substance to dissolve in another substance and form a solution. Sugar dissolves well in water, whereas oil does not.

Thermal conductivity is the ability of certain substances to conduct or transfer heat. Metals, such as iron or copper, are good heat conductors, while materials such as wool, wood, paper, plastic or glass are good heat insulators.

Mass is the amount of matter in an object. An apple has a greater mass than a grape. We measure mass in grams (g).
1 000 grams = 1 kilogram

WHAT TYPES OF MATTER ARE THERE?

According to its composition, we can classify matter into pure substances and mixtures.

• Pure substances, such as table salt or silver, consist of only one type of matter.
• Mixtures are made up of two or more pure substances and can be homogeneous or heterogeneous.
• In a homogeneous mixture, such as the air we breathe or sea water, we cannot see the individual substances that make it up.
• In a heterogeneous mixture, such as sand or a salad, we can see the individual substances that make it up.

There are different methods to separate the substances in mixtures.

Filtration

When we pour the juice of a lemon through a filter, the solids stay in the filter, and the liquid passes through it.

Evaporation

When we heat a mixture of salt and water, the liquid evaporates and leaves only the solids which crystalliseand form salt crystals.

Sieving

We use a sieve to separate solids of different sizes, such as pebbles from sand.

Forces and their effects

A force is a push or pull that acts on an object. We can't see forces, but we can see and feel their effects. Forces can make things move, stop, speed up, slow down, or change direction. Forces can also make things change shape.Some forces act from a distance. These forces are called non-contact forces. Other forces act through physical contact. These are called contact forces.

Non-contact forces

Magnetism is a force of attraction or repulsion caused by magnetic materials. Magnets attract metals and also make compasses work.

Gravity is a force which attracts all objects towards each other. Larger bodies, such as planets, have a stronger gravitational force. Gravity stops everything on Earth from flying off into outer space and also causes objects to fall to the ground. Astronauts float in space, because there is no gravity there.

Contact forces

Friction is the force that exists when two objects rub against each other. Friction acts in the opposite direction to the direction of movement. It can slow down or stop a moving object. Air friction slows down a parachute and water friction slows down a kayak or a boat.

Buoyancy is the ability of objects to float. It is an upward force which acts in the opposite direction to gravity on objects immersed in a liquid. If you put an ice cube in a glass of water, gravity pulls the ice cube down and the buoyant force pushes it up. The submerged ice cube displaces some of the water and, as a result, the level of water rises.

Why some objects float in water and others sink?

Density explains why some objects float in water while others sink. Look at the densities of cork, water and iron.

The density of steel is around 7.7 to 8.0 kg/l. Why do ships float if they are made of steel which has a bigger density than water?
The ability to float depends on the average density of the object. Average density takes into account the steel that the ship is made of and the air that is trapped in the body of the ship.

A ship with a lot of air inside has a lower average density than water, so it floats.

A ship with cargo has less air inside, but its average density is still lower than the density of water. That's why it still floats.