
π Friction: Summary Note for MCQ
πΉ 1. What is Friction?
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Friction is the resistive force that opposes the relative motion or tendency of motion between two surfaces in contact.
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It acts parallel to the surfaces in contact and opposite to the direction of motion.
πΉ 2. Types of Friction
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Static Friction:
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Acts when the object is at rest.
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The highest value before motion starts = Limiting friction.
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Variable in nature: 0β€fsβ€fmax0 leq f_s leq f_{text{max}}
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Kinetic (Sliding) Friction:
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Acts when the object is in motion.
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Constant value less than limiting friction.
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Rolling Friction:
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Occurs when an object rolls over a surface.
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Smallest among all types.
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πΉ 3. Cause of Friction
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Due to irregularities or microscopic roughness of the surfaces in contact.
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Even polished surfaces have microscopic irregularities.
πΉ 4. Laws of Friction
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Friction is independent of the area of contact.
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Friction is proportional to the normal force:
f=ΞΌNf = mu N
Where:
ff = Frictional force
ΞΌmu = Coefficient of friction
NN = Normal reaction -
Limiting friction depends on the nature of the surfaces in contact.
πΉ 5. Coefficient of Friction (ΞΌ)
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Ratio of limiting friction to normal force:
ΞΌ=fNmu = frac{f}{N}
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It has no unit (dimensionless).
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ΞΌβ > ΞΌβ > ΞΌα΅£
(Static > Kinetic > Rolling)
πΉ 6. Advantages of Friction
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It helps with walking, writing, gripping, driving, etc.
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Essential for brakes, machines, and daily motion.
πΉ 7. Disadvantages of Friction
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Causes wear and tear of machines.
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Produces unwanted heat and energy loss.
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Reduces the efficiency of moving parts.
πΉ 8. Methods to Reduce Friction
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Polishing surfaces
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Lubrication (using oil or grease)
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Using ball bearings or rollers
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Using streamlined shapes (for fluids/air)
π§ Quick MCQ Facts
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Friction is a contact force.
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Static friction is self-adjusting; kinetic is constant.
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Friction depends on normal force, not on surface area.
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Rolling friction is always less than kinetic and static friction.
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Friction always opposes relative motion or its tendency.