1. Why is rolling friction less than sliding friction? (Hint: Draw an illustration of the surfaces in contact
in both cases. In which case is the area of the contact surface greater?)
2. Can you think of a situation where it is necessary to increase air resistance on a falling object? How
is it done?
3. Water on a smooth floor can make you slip. This means water is also a lubricant. Can you think
of reasons why oil and grease, and not water, are used in machines as a lubricant? (Hint. Think of
evaporation and corrosion.)
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No, friction does not always oppose the motion. When we walk on the ground, we push the ground in the backward direction. Then it is the friction force between the ground and us, which helps us moving forward. Also, while taking a turn on a circular road, friction force works as a centripetal force and helps in changing the direction.
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