What are the differences between solid friction and viscosity?
Difference between solid friction and viscosity is as follows:
Viscosity is proportional to the surface area whereas solid friction is independent of area of solid surfaces in contact. Viscous force on the body depends upon its velocity in the viscous media. But, friction does not depend on the velocity of the body.
The wings of the aircraft have upper side convex outside and lower surface concave inside. This causes the air that flows over the top of the airfoil to move faster than the air that flows beneath it.
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How does the raindrop attain constant velocity after some time?
The stoke's law states that the viscous force acting on the body is directly proportional to its velocity. The velocity of the falling raindrop increases due to gravity acting on it. So, the viscous force also increases. After a certain amount of velocity is attained, the weight W becomes equal to the upward forces. That is, the viscous force and upward thrust of the raindrop acquires a constant velocity.
[Note: According to stoke's law, the backward dragging force opposes the motion of the body. And, backward dragging force increases with increase in velocity of the moving body.]
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Why machines are sometimes jammed in winter?
The machines are lubricated with oils to reduce the force of friction. In winter, due to fall in temperature the viscosity of oil increases and hence machines get jammed.
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Hotter liquids move faster than colder ones. Why?
With the increase in temperature the coefficient of viscosity decreases. So, the velocity of the flowing liquid increases. Therefore, hotter liquids move faster than the colder ones.
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