Name: friction’s role in machine efficiency
Uploaded: 2023-09-22T11:49:20.000Z
Duration: 1 min 14 s
Channel: JoVE

L'efficacité mécanique d'une machine est le rapport entre la production et le travail d'entrée.
Pour une machine idéale, le rapport est de un, ce qui implique que le travail d’entrée et de sortie est égal.
Par exemple, dans un étau à genouillère idéal qui comprime le bois, le travail de sortie dû à la force de réaction est égal au travail d’entrée dû à la force appliquée.
En revanche, une vraie machine subit une force de frottement.
La force de frottement peut être obtenue à partir de la force de réaction normale.
Le travail virtuel total dû aux forces contributives, lorsqu’il est appliqué dans le principe du travail virtuel, donne le travail de sortie comme la différence entre le travail d’entrée et le travail dû à la force de frottement.
La division du travail de sortie par le travail d’entrée donne l’efficacité mécanique. L’efficacité en l’absence de friction est un. Cependant, pour un coefficient de frottement non nul, l’efficacité est nulle lorsque l’angle de bascule est égal à l’angle de frottement et négatif pour les angles de bascule inférieurs à l’angle de friction.
Cela implique qu’un étau ne doit être utilisé que pour un angle de basculement supérieur à l’angle de frottement.

<ol>
	<li>Beer, F.P., Johnston, E.R., Mazurek, D.F., Cornwell, P.J. et Self, B.P. (2016). Mécanique vectorielle pour les ingénieurs. Onzième édition, New York : McGraw-Hill Education. Pages 580 et 581.</li>
</ol>

efficacité mécanique des machines réelles

L'efficacité mécanique d'une machine est un concept fondamental qui décrit comment une machine peut convertir efficacement le travail d'entrée en travail de sortie. Selon ce concept, l'efficacité d'une machine est égale au rapport entre le travail de sortie et le travail d'entrée. Une machine idéale, c'est-à-dire une machine sans pertes d'énergie, a une efficacité d’un. Cela implique que le travail d'entrée et le travail de sortie sont égaux. Cependant, en réalité, aucune machine ne peut être réellement idéale, et toutes subissent une forme de perte d'énergie. L'une des principales sources de perte d'énergie dans les machines est la friction. Par exemple, considérons un étau à bascule utilisé pour comprimer des blocs de bois. En l'absence de friction, le travail de sortie de l'étau est égal au travail d'entrée de la force appliquée, ce qui en fait une machine idéale. Cependant, en réalité, le bloc coulissant de l'étau subit une force de frottement contre le plan horizontal. Pour déterminer l'efficacité mécanique d'un étau réel, calculez d'abord la réaction normale et la force de frottement correspondante. Évaluez ensuite le travail virtuel total de toutes les forces pendant le déplacement virtuel. En appliquant le principe du travail virtuel, le travail de sortie de l'étau se révèle être la différence entre le travail d'entrée et le travail de la force de frottement. Par conséquent, la force de réaction nette peut être calculée, qui est nulle lorsque l'angle de l'étau est égal à l'angle de frottement et négative lorsqu'il est inférieur à l'angle de frottement. Cela signifie que l'étau ne doit être utilisé que lorsque son angle dépasse l'angle de frottement. L'efficacité mécanique de l'étau en l'absence de friction est d’un. Cependant, lorsque la friction est présente, l'efficacité peut devenir nulle ou inférieure à un, en fonction du coefficient de frottement. Plus le coefficient de frottement est faible, plus l'efficacité sera proche d’un – inversement, plus le coefficient de frottement est élevé, moins l'efficacité est élevée. L'efficacité mécanique d'une machine est un concept essentiel qui explique comment une machine peut convertir efficacement le travail d'entrée en travail de sortie. En réalité, aucune machine ne peut être entièrement idéale et elles subissent toutes une forme de perte d'énergie due à la friction. Lors de l'analyse de l'efficacité d'une machine, il est crucial de prendre en compte les effets de la friction et de son coefficient car ils jouent un rôle significatif dans la détermination de la force de réaction nette et de l'efficacité globale de la machine. Alors qu'une machine idéale peut atteindre une efficacité parfaite, les machines réelles auront toujours une efficacité inférieure à l'efficacité idéale en raison des pertes d'énergie, notamment de la friction.

Efficacité mécanique des machines réelles

L’efficacité mécanique d’une machine est un concept fondamental qui décrit l’efficacité avec laquelle une machine peut convertir le travail d’entrée en travail de sortie. ...

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Mechanical Efficiency of Real Machines

friction’s role in machine efficiency

Mechanical efficiency shows how well a machine turns input work into output work. It is calculated as the ratio of output work to input work. An ideal machine has an efficiency of one, which means the input work equals the output work.

Real machines are never perfectly ideal because some energy is lost. Friction is one of the main causes of that loss. In a toggle vise used to compress wooden blocks, the sliding block rubs against the horizontal plane and creates friction.

To find the efficiency of a real vise, first calculate the normal reaction and the frictional force. Then use the principle of virtual work to evaluate the total virtual work of all the forces during a virtual displacement. With friction present, the output work equals the input work minus the work done by friction.

This analysis also gives the net reaction force. It is zero when the angle of the vise equals the angle of friction. It becomes negative when the angle is smaller than the angle of friction, so the vise should be used only when its angle is greater than the friction angle.

A vise with no friction has an efficiency of one. When friction is present, the efficiency can be less than one or even zero, depending on the coefficient of friction. A lower coefficient of friction gives an efficiency closer to one, while a higher coefficient lowers the efficiency.

Friction’s Role in Machine Efficiency

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key terms and concepts

Learn how friction lowers mechanical efficiency in real machines, using a toggle vise example, virtual work, and the angle of friction.

Friction’s Role in Machine Efficiency | JoVE K12

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Friction’s Role in Machine Efficiency

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