Science Helpdesk

A force is a push, a pull or a twist which is exerted in an specific direction. Forces are measured with balances. The small ones can be measured with a spring balance (= forcemeter). Their values are expressed in newtons (N).

Forces can have several effects upon the objects: they can (a) change their shape, (b) change their direction if they're moving; and (c) change their speed by slowing them down, speeding them up or putting them into movement.

The movement of an object in any medium (except the vacuum) produces automatically a force in exactly the opposite direction: this is the force of friction (of the air, the water, the ground). This force slows down the moving object and tends to ultimately stop it. It requires the moving object to use more energy to overcome the friction and reach the wanted speed.

But the force of friction doesn't only imply a higher use of energy: it can also be useful. For instance: it is the force of friction generated when you push your feet backwards against the ground, the one that, in return, pushes you forward when you are trying to run. And it is the force of friction done by a rubber pad against a wheel the one that makes a bike or a car slow down when you press the brake.

When two forces are acting on an object in opposite directions they can...

• Be equal in size: these are balanced forces and cancel each other out, i.e., the resultant force is zero. One example is when the force of friction of the air and the ground (called drag) cancel out the driving force of a car (produced by the engine thrust). In this case the car moves at a steady speed, called terminal speed. Another example is when you are standing still on the floor: you know that the force of gravity is always pulling you downwards, so to keep you there, instead of being swallowed by the Earth, the floor must be doing a force upon you exactly the same size than the gravity force, but in the opposite direction.
• Be different in size: these are unbalanced forces and do not cancel each other out. They change the motion of the object, which (a) will start to move in the direction of the bigger force, if the object was still; (b) will move faster, if the object was moving in the direction of the bigger force; (c) will slow down, if the object was moving opposite to the bigger force.

Gravity is a force. It is the force that pulls objects together. It is bigger the bigger and the closer these objects are. The gravity force between two small objects or between two very distant stars isn't perceptible, but it is clearly noticeable between the Earth and every object on its surface, because every object has a weight, and this is the force with which the Earth pulls any near object downwards.

Therefore, your weight will be bigger (a) the closer you are to the centre of gravity of the Earth (which is the centre of the Earth) and (b) the bigger you are, or more exactly, the more mass you have. Your mass is the amount of matter you have (atoms and molecules), and with the same mass you can have different weights: your weight will be smaller in the stratosphere, because you are further away of the Earth's centre of gravity, or in Mars, because it has about 8 times less mass than the Earth: in both cases the gravity acting on you is smaller than in the surface of the Earth. But your weight would be bigger in Saturn or Jupiter, for the opposite reason: there is a bigger gravity acting on you now.

As weight is a force, it is measured in newtons. In the Earth, at sea level, the numeric value of your weight, in newtons, is roughly ten times your mass. In the Moon, you have to multiply your mass by only 1.7 to find how many newtons you weight. And in the outer space, far away from the gravitational action of any celestial body, your weight would be almost zero. In the three cases you would have exactly the same mass.

Speed is the rate at which an object covers a distance. You calculate it dividing the space covered by an object, by the time taken on it. Its unit in the SI is m/s.

Distance-time graphs are useful to represent the variation of the distance covered by an object in every unit of time, i. e., they are useful to represent visually the speed of an object. When the line in the graph shows a steady slope (when it is a straight line), the object has a steady speed; otherwise, the graph will indicate some sort of positive or negative acceleration. Also, a steep slope will express a higher speed than a gentle slope. And a straight horizontal line will show that the object is stopped.

If you plot a speed-time graph you will represent visually the acceleration of an object. In this case, a straight horizontal line shows acceleration zero, i. e., the object is moving at a steady speed. A straight line with a positive slope will represent an object speeding up. And a straight line with a negative slope will represent a negative acceleration: the object is slowing down. If you want to represent a stopped object in this kind of graph, you have to draw a straight line along the X axis.

The goal of power plants is to produce electricity that is later dumped into the general supply network. That electricity is produced by an electrical generator driven by a turbine, in all cases. The differences between the several types of power plants come with the way in which that turbine is moved:

• In the hydroelectric power stations, it is the water flow of a river what moves the turbine.
• In an aerogenerator it is the wind what moves the turbine (which consists of a set of three big blades).
• In the rest of the cases it is water vapour at a very high pressure what moves the turbine. That water vapour is produced by heating water, and the heat can come from…
• igniting coal or other fossil fuels or biomass (in thermal power plants);
• breaking nuclei of uranium or plutonium (in nuclear power plants).

This way, the production of heat in a power plant goes through a series of stages in which a transfer and a transformation of energy take place. The following diagram summarizes what happens in a coal-fueled thermal power plant:

• Chemical energy in the coal -> Thermal energy in the water vapour -> Kinetic energy in the turbine -> Electric energy in the generator and to the supply network wires

Physical changes are those in which the substances that make up a physical body remain the same when the body undergoes a change.

A physical change can be produced…

• When you apply a force to an object, as when you crumble a piece of paper with your hand;
• When you give or take energy to/from an object, as when you heat up or cool down water.

Some examples of physical changes are…

• A change of state, such as the condensation of water;
• A change of temperature, as the cooling down of food inside the fridge;
• A change in the shape, as when you shatter mom's favourite china vase;
• A change in the amount or type of energy in an object: so happens when you drop a stone to the floor.

Chemical changes (or chemical reactions) are those that transform some chemical/s into new other chemical/s.

They normally occur when different substances, prone to react between them, are put into contact. And so, when iron (Fe) and oxygen (O₂) are put together, they will combine and disappear to form a new chemical: rust (Fe₂O₃). But the wine and the nitrogen (kept in the upper part of the bottles of wine) will never react, allowing the wine to preserve its properties over time. Not all chemicals will react when put into contact.

The initial molecules that react in a chemical reaction are called reactants, and the final resulting molecules are called products. In the previous reaction, the oxygen and the iron are the reactants, whereas the rust is the only product.

Some chemical changes imply a loss of energy, i.e., they are exothermic, because they release energy (usually heat) to the environment. Some other chemical changes are endothermic, they absorb energy from the environment, and the products will have a greater amount of energy than the reactants.

Nevertheless, most chemical reactions, either exothermic or endothermic, require an initial contribution of external energy to take place. If you want a piece of paper to combine with the atmospheric oxygen and go into combustion, you need to heat up the paper (what you usually do with a flame). But during the reaction, the paper releases a lot of energy (heat, light and even sound), so the reaction is exothermic: the paper and the oxygen had more energy than the ashes, the smoke, the CO₂ and the water vapour produced.

Quite often, chemical changes can be noticed by some conspicuous events, such as…

• A change in the temperature, as in any combustion;
• A change in the colour, as in the rusting of iron;
• The formation of bubbles, as when the calcite (CaCO₃) reacts with hydrochloric acid (HCl);
• A change in the volume, as in baking bread;
• Etc.

Most chemical changes are irreversible: they can't be undone (e.g. a combustion), whereas most physical changes are reversible (e.g. the condensation of water).

Chemical equations are the way by which chemical reactions are represented. To the left you write the formulas of the reactants (either elements, molecules or crystals) and to the right, the formulas of the products. In between, you draw an arrow that represents the direction of the change: from the reactants to the products. In case of a reversible reaction, you must draw a double arrow.

One example would be the following:

• 3 H₂ + N₂ → 2 NH₃

The number of molecules of each type is represented with a coefficient to the left of the molecule. The number of atoms for each kind of molecule is represented by a subindex to the right of its chemical symbol (nothing when it's just one). Thus, you can see that in the equation above we have six atoms of hydrogen and two of nitrogen in each side. This is because in a chemical reaction, neither the chemical elements, nor the number of atoms of each chemical element change. What changes is the arrangement of those atoms because a chemical change actually consists of the breaking down of some chemical bonds and the formation of some new chemical bonds.

The following chemical equation does not represent any real chemical reaction, because there aren't exactly the same atoms in each side of the equation:

• H₂ + N₂ → NH₃

Also, as the atoms of the reactants have to be the same as the atoms of the products, the overall mass does not change during a chemical reaction.

Balancing a chemical equation means writing the right coefficients to the right of each molecular species, to ensure that the number of each type of atoms (and so, the overall mass) remains the same along the equation.

Metals with oxygen and water

Metals react with oxygen and water depending on their reactivity. This means that some are highly reactive, some others are moderately reactive, and some others are very little reactive or even nothing at all. The following list shows the reactivity of some common metals: K > Na > Ca > Mg > Al > Zn > Fe > Sn > Pb > Cu > Ag > Au > Pt.

This way, while potassium reacts violently with O₂, iron forms rust (Fe₂O₃), silver slowly tarnishes (forming Ag₂O), and gold does not react at all and preserves its properties forever.

Likewise, as combustions are oxidations boosted by heat, the behaviour of metals when they are heated varies: some burn (Mg) while others just rust (Cu).

Metals with acids

Metals react with acids forming hydrogen gas (H₂) and a salt. This reaction consists of the displacement of the hydrogen of the gas by the metal, as this one is more reactive than the hydrogen.

One example is sodium reacting with hydrochloric acid to form sodium chloride (table salt) and hydrogen:

• Na + HCl → NaCl + H₂

This type of reactions can be tested by testing the formation of H₂, which is a gas that is able to put out the flame of a match.

Displacement of metals by metals

As some metals are more reactive than others, the more reactive metals can displace the less reactive metals from their compounds. For instance: magnesium is more reactive than iron, and so it can displace the iron from the ferric oxide:

• 3 Mg + Fe₂O₃ → 3 MgO + 2 Fe

For the same reason, platinum would never displace any other metal from its compounds, as it is the least reactive metal of all.

These type of reactions are useful to purify metals that are rarely found in pure state in nature. This is the case of iron, that usually occurs forming the minerals hematite (Fe₂O₃) and magnetite (Fe₃O₄). Aluminium can be used to displace iron from its oxides, and thus, purify it:

• 2 Al + Fe₂O₃ → Al₂O₃ + 2 Fe

Acids and alkalis

Acids and alkalis are common everyday substances, such as acetic acid (in vinegar), citric acid (in lemon juice), caustic soda (lye, NaOH) or bicarbonate of soda (baking soda).

Acids and alkalis have a very different behaviour in solution: while acids release protons (H⁺), alkalis sequestrate the protons. The amount of free protons is measured with the pH scale: the greater the amount of free protons, the lower the pH, and the lower the amount of free protons, the higher the pH. Very acidic solutions have a pH of 1, whilst very alkaline solutions have a pH of 14. Neutral solutions and distilled water, neither acidic nor alkaline, have a pH of 7. Salts make neutral solutions; such is the case of a solution of table salt in water.

Indicators, such as litmus paper or a solution of phenolphthalein, can tell us whether a solution is acidic or alkaline: litmus paper or litmus solution will turn red in an acidic solution, and blue in an alkaline solution. Universal indicator is more precise, though: it is a mixture of dyes that can be present in paper strips or in solutions, and it yields a wide range of colours for the whole pH scale: from red (very acidic solutions) to green (neutral solutions) and to purple (very alkaline solutions).

Reactions between acids and bases

Alkalis are soluble substances that belong to a group of chemicals called bases. Bases react with acids producing a salt and water. These are called neutralisation reactions, as both the acid and the base "disappear" to form a salt, which gives a neutral solution

Some examples of everyday neutralisation reactions are the following:

• Treating bee stings (acidic) with sodium bicarbonate (a base);
• Treating wasp stings (alkaline) with vinegar (that contains acetic acid);
• Upping the pH of an acidic soil with lime (CaOH, an alkali);
• Neutralising the excess of HCl that your stomach produces during digestion with antacids such as baking soda (a base).

The reactions between acids and bases can produce a wide range of different salts. For example:

• Salts of sulphuric acid are called sulphates:
•     H₂SO₄ (sulphuric acid) + 2 NaOH (caustic soda) → Na₂SO₄ (sodium sulphate) + 2 H₂O

• Salts of nitric acid are called nitrates:
•     HNO₃ (nitric acid) + NaOH (caustic soda) → NaNO₃ (sodium nitrate) + H₂O

• Salts of hydrochloric acid are called chlorides:
•     2 HCl (hydrochloric acid) + CaCO₃ (calcium carbonate) → CaCl₂ (calcium chloride) + H₂O + CO₂

Living matter is the one able to carry out the three so called vital functions:

• Nutrition, which consists of taking in matter and energy in order to grow, survive and reproduce; waste matter and waste energy are produced as by-products. If a living being feeds directly off other living beings (such as animals, fungi and protozoa) it is called a consumer or heterotroph; but if it takes the matter and the energy that it needs from the inert matter (such as plants and algae) then it is a consumer or autotroph. Another way to express this difference is that consumers take in both organic and inorganic molecules, while the producers only feed on inorganic substances.
• Interaction is (a) the ability to perceive what is going on in both the environment and the inside of the organism itself, and (b) the ability to produce responses coherent with the information that was perceived. It usually goes as follows: an stimulus is perceived by a receptor → a control centre analyses the stimulus and generates a response order → an effector performs the response.
• Reproduction, the ability to produce living beings similar to the parental organisms. It may be sexual (when there is only one parental organism, as in bacteria) or sexual (when two different types of individuals, male and female, are required).

Every living being, and every cell in the multicellular living beings, is able to carry out these three vital functions.

The Ecosystems

One ecosystem is any area that happens to have an specific set of features (living species and physical-chemical conditions) that make it significantly different from its surroundings. Thus, a forest is an ecosystem, but a lagoon or a puddle of water in that forest, are themselves ecosystems too. A city is also an ecosystem: a man-made ecosystem.

The limits of many ecosystems are difficult to define. For instance, a lagoon in a forest can be fed with water from an aquifer that extends throughout an area much bigger than the forest itself, and that also feeds with water the farm-land that surrounds the forest. This way, the lagoon, the forest and the farm-land are connected. Ecosystems are not isolated in Nature: there are connections and transitional areas between them.

When an ecologist tries to describe an ecosystem, he/she has to describe the following sorts of facts:

• The set of living species in it, that is, the biocenosis. For instance, in a forest there could be pines, oaks, mice, doves, butterflies, kites, ants…
• The set of physical and chemical conditions, that is, the biotope. In a forest: the average temperature in the coldest month, the average temperature in the hottest month, the rainfall rate, the kind of salts that exist in the soil…
• The relationships between the living species: the pines holding the nests that doves make, the kites eating the mice, the mushrooms feeding on the fallen leaves…
• The relationships between the physical-chemical conditions: how temperatures alter the soil's humidity through evaporation, how wind wears away soil particles…
• The relationships between the physical-chemical conditions and the living species: the way in which the animal activity is affected by the day-night cycle, the way in which temperature affects the loss of water vapour by plants through evapotranspiration, the way plants retain the soil particles with their roots and prevent them from being worn away by the wind, the way in which the decomposition of fallen leaves or dead animals adds new salts and minerals to the soil…

The Ecosphere

Every ecosystem can have other ecosystems inside of it. The Earth itself is one ecosystem and comprises all the other ecosystems. The Ecosphere is the name we give to our planet when we think of it as an ecosystem, and it has a biotope and a biocenosis. The biotope of the Ecosphere is made up by the Geosphere, the Hydrosphere and the Atmosphere. The biocenosis of the Ecosphere is called Biosphere, and comprises all the living beings of the Earth.

The Biomes

If we wanted to split the Ecosphere in the biggest possible ecosystems, we would split it into the biomes. A biome is a large ecosystem made up of all the ecosystems belonging to the same climatic zone. There are, for instance, the tropical rainforest biome, the mediterranean forest biome and the desert biome.