Lesson 7, Topic 6
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# Similarities & Differences between Viscosity and Friction

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### Similarities Between Viscosity and Friction:

1. Viscosity and friction both oppose relative motion between two surfaces.

2. Both depend on the nature of the materials in contact.

### Differences Between Viscosity and Friction:

1. Friction is dependent on normal reaction, viscosity is not.

2. Friction is independent of areas of surfaces in contact, viscosity depends on the area of surfaces in contact.

3. Friction occurs in solids, and viscosity occurs in fluids (liquids & gases).

4. Friction force does not depend on the relative velocities between the two layers in contact while viscosity depends on the relative velocities of the two layers in contact

### Application of Viscosity:

1. Oil, grease, and air are used as lubricants because of their viscosity. The use of lubricants in engines helps to prevent the metal surfaces from rubbing together.

In liquid, viscosity reduces with an increase in temperature because a larger temperature means particles have greater thermal energy and are more easily able to overcome the attractive forces binding them together. This decreases the resistance to flow through the liquid. Due to this reason, the lubricating effects of oil and grease are lowered at high temperatures. Because of water’s low viscosity, it is not used as a lubricant.

Note: In the case of gases, viscosity increases with an increase in temperature because the velocity at which the gas molecules collide with one another increases. This leads to an increase in resistance to flow, thereby increasing viscocity.

2. Viscosity of air makes the Bob of a swinging pendulum come to rest.

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