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.