Frequently people who don't really understand why perpetual motion machines
won't work, give the simple explanation: "because of friction". This explanation
is convenient and seems to be a good excuse not to dive into the details
of a particular mechanism. However, it is a poor explanation, as it ignores
the fact that in physics, friction frequently is not taken into account to
make theory and explanations easier. And what about all magnet and electric
machines, which may have not moving parts at all?
Here, I will take frictional effects in account only, if they are a vital
point in an argument chain. In this case, it is the best method to do the
analysis without friction first and then add the friction in correction terms.
In practice, friction is important and cannot be ignored, as friction is the reason for losing part of the energy by transforming it into heat. Normally, we associate with the word friction losses in bearings or sliding machinery parts. But friction lurks everywhere. Here are some examples which might need to be considered if we want to have a more detailled look into machines:
| Type | ...and something about it |
|---|---|
| sliding friction | Whenever a sledge is pulled over the ground or a sliding bearing in machinery is applied, two material surfaces rub on each other. As surfaces are not perfectly smooth, the work needed to overcome the friction is transformed into heat, which can be very high, e.g. when metal tools are ground on a stone. This type of friction can be reduced, but not eliminated by applying a suitable type of oil or grease to the sliding surfaces. Air cushions or the repelling effect of magnets can also reduce sliding friction. But keep in mind that in all these cases, one type of frictional behaviour is replaced by another type which also must be taken into account. |
| rolling friction | Wheels rolling over the ground allow less work to overcome friction as if the same mass would be pulled on a sledge. Because of this, engineers frequently prefer roller bearings in order to reduce frictional losses. |
| elastic deformation | Steel balls bouncing on a stone floor demonstrate elasticity. Both stone and steel are hard materials and to a certain extent elastic. However, after a while, the steel balls come to a rest. The reason is not only air resistance, but an inner deformation work which consumes the mechanical energy and transforms it into heat. |
| plastic/inelastic deformation | Whenever a soft material like clay or lead undergoes deformation due to external forces, it will not automatically regain its original shape. The work applied to change the shape of the mass will be transformed into heat. |
| liquid friction | Depending from the viscosity of a liquid, the inner friction can be considerable. Inner friction in liquids and gases is the reason for mechanical resistance, when objects are moved through these media. Consequently, objects which are to be moved are streamlined in a suitable way. |
| electromagnetic friction | An effect which is very significant if a non-magnetic conducting material like copper is moved in a magnetic field. This effect is practically used for braking of trains or engines. Unintendedly, it causes energy losses by heating up the moved material due to short-circuit currents inducted in the material. |
| tidal friction | An effect which is frequently underestimated. Followers of perpetual
motion often refer to the celestial sphere as an example of mechanical perpetual
motion. A motion which is not so perpetual as we think. Have you ever wondered
why the moon always shows the same face? The reason is tidal friction. The
tidal forces caused and cause these effects:
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| sound | Machines which are noisy, waste energy. Noise is caused by several types of friction. Additionally, sound itself can cause losses. These may occur inside an object by reflections of sound waves or by emitting vibrations into the surrounding atmosphere. |
A few lines above, I wrote that friction lurks everwhere. To give you an
impression, at which places, imagine a loaded wheelbarrow and assume, you
have to push it on level ground through your garden.
Normal abstraction in physics would state that no work is needed to transport
the load alongside the path, as this is done on level ground. I assume, you
won't agree. The reason is: friction! We will find these types of friction
if we do a closer analysis of the situation.
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All these effects sum up to the undesired result that you definitely have to work, if you want to push the wheelcart.
I dislike this expression. The word implies that friction causes forces and
moreover gives the misleading impression that "frictional force" multiplied
by distance is work. Friction can't generate work. Friction can't even generate
force. Friction can only cause mechanical resistance in terms of Newton's
famous action equals reaction principle. And so you cannot expect
that some type of a mechanism, heavily burdened with friction causes a force
that can be applied for useful work.
At least one inventor misunderstood this concept completely. Around 1920
the czech F. Prachar suggested a variant of an overbalanced wheel powered
by frictional forces!
Often, friction is considered as being an unwanted accessory in machines.
But friction is very important, as it allows us to walk, to drive a car or
to apply a brake in machinery. Friction is the reason why our shoes don't
slip over the ground. If you had ever tried to walk on ice, you can imagine
how useful friction is. Which examples do you know, where friction is an
inherent part of the functionality?
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A useful application which was invented by an engineer named Prony is the so-called prony brake. It's a device to measure and calculate the power a machine delivers at its output shaft. |
| Last update: 4 July 2003 / |
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