Adhesion is the phenomenon that occurs when two surfaces tend to stick together, therefore preventing sliding.
When a car stops in the middle of a hill, the tires allow the vehicle to stay in place without slipping. When a person climbs a wall, their shoes have a special sole allowing them to grip the rocky surface without slipping. These phenomena, which allow objects to stay in place and prevent sliding, are examples of adhesion. Without adhesion between the surfaces in contact, the car or the climber would not be able to stay in place.
The soles of climbing shoes provide a grip on the rock.
The rubber of the tires allows the car to stay in place.
The adhesion force between two surfaces depends on five factors.
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The nature of the materials in contact: The adhesion between rubber and asphalt is greater than the adhesion between rubber and ice, which explains why a car does not hold the road as well in winter.
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The presence or absence of a lubricant: The wax applied to cross-country skis decreases their grip on the snow and therefore improves their sliding.
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Temperature: The adhesion of shoe soles on asphalt decreases when the temperature drops, which makes our balance less stable in winter.
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The condition of the surfaces in contact: The rougher a surface, the more it is likely to adhere to another surface. So, the tires of a road bike are smoother than those of a mountain bike in order to decrease the grip and consequently to increase the rolling speed.
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The perpendicular force exerted by one surface onto the other: The adhesion increases as this force increases. Therefore, it is more difficult to pull a loaded sled over snow than an empty one.
In a technical object, the adhesion between the surfaces is important to ensure that certain links are made between parts or that certain motion transmission systems are efficient.
Friction is a force that opposes the sliding between surfaces in contact.
Unlike adhesion, friction between surfaces does not prevent parts from moving. However, by hindering motion, it leads to premature wear and loss of energy. To decrease the friction between parts, a lubricating component is usually added to the technical object.
There are always friction forces between two surfaces in contact. When these forces are large enough to prevent one surface from sliding over another, it is called adhesion.