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.