Why do you think that car engines have to have oil put in them? Why do door hinges have to be oiled every now and then, why do we have brakes on cars, motorcycles and pedal cycles?


In a perfect world, we wouldn't need any of this. Things would just move along without any need for anything to make it happen. For example we wouldn't need an engine in the car to push the car forward, it would just go forward on its own, so why doesn't it?


Frictional forces, often we shorten this to just "friction" is the reason for this, and it's probably a good thing that it exists anyway. You are sitting on a seat (I would imagine anyway, reading about this amazing subject) and it is friction that is keeping you there, without friction you would slide off the seat and slide along the floor, only stopping when you touch the wall or some other object.


So what exactly is friction? Well.... It is a force which acts in the opposite direction to movement. If you are cycling on a flat surface and you stop pedalling, you will eventually slow down and stop (assuming that you didn't use the brakes first). The reason for this is the effect of friction of the rubber in your tyres on the surface. To keep moving along you have to provide a "driving force" to counteract the effect of friction otherwise you would simply keep stopping. Of course it isn't only the effect of the tyres that would slow you down, air resistance acting against you will also slow down your movement to a standstill unless you pedal to keep moving.


Terminal Velocity


If you jumped out of an aeroplane with a parachute strapped to your back, you would increase in speed (or more correctly velocity) until you reached a certain point. You won't just keep getting faster and faster, and if you think about it you'll probably be able to work out for yourself why.



When you initially jump out of the plane, the acceleration due to gravity (the force of gravity if you like) is much stronger than the air resistance trying to slow you down. But as you continue falling the air resistance increases until it balances with the effect of gravity. At this point you will still continue to fall of course, but you will not get any faster. This is known as your terminal velocity. When you deploy your parachute the massively increased air resistance of the parachute will slow you down considerably (this is what parachutes are designed to do).


Air resistance is also known sometimes as "drag" and motor-vehicle manufacturers are constantly striving to develop more streamlined cars with a reduced drag. The more that the effect of air resistance can be reduced, the more economical it will be to run the car because less fuel will have to be used to provide the driving force.



In the picture above, we can see that there is a driving force pushing the car forward and a resistive or "drag" force trying to hold it back. The forward facing black arrow is longer than the rearward facing red arrow which suggests that the forward force is greater than the backward force. As a result of this the car will move forward. Also note that the force of gravity (acceleration due to gravity) is pulling down on the car but this is counterbalanced by the car pulling back. If the downward force was greater than the upward force, the car would disappear into the ground, similarly the upward force was greater than the downward force the car would float upwards! It is important that these particular forces are balanced, and we are usually only concerned with the "horizontal" forces.


The whole subject of whether or not forces are balanced brings us nicely into the three laws of Sir Isaac Newton.