About Friction

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:
  • The moon was braked down due to the inner friction of the semi-liquid core, until the tidal force was no longer effective. This is exactly the case when the moon's rotational period is identical to the time it needs to revolute once around the earth.
  • The tidal forces of the moon partly heat the inner earth because of inner friction of the earth's core. Because of this, and the friction by the tidal water-hills, the rotational speed of the earth slows down.
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 Friction Example

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.
  • Rolling friction of the wheel on the ground
  • sliding friction in the bearing of the wheel's axle
  • liquid friction in the bearing's oil
  • elastic deformation of the tyre's rubber
  • Inner friction in the rubber material due to hysteresis
  • Plastic deformation of the soft soil
  • air friction due to the movement of the wheelcart (can normally be negleted in this situation)

All these effects sum up to the undesired result that you definitely have to work, if you want to push the wheelcart.

 Frictional Force

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!

Useful Friction

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?
   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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