Questions and Answers
What is the relationship between static friction and the normal reaction force on a horizontal surface?
The coefficient of kinetic friction is always greater than that of static friction.
False
What is the relationship between the angle of repose and the angle of friction?
They are equal.
The formula to calculate kinetic friction is __________.
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If a block has a weight of 20 N and starts sliding with a tension of 8 N applied, what is the coefficient of static friction?
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For a body moving down an inclined plane, static friction is always acting in the direction of motion.
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What condition must be met for kinetic friction to occur?
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Match the terms to their definitions:
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What is the relationship between limiting friction and normal reaction?
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Kinetic friction is always greater than limiting friction.
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What is the coefficient of static friction denoted by?
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The value of limiting friction is independent of the ______ of the surface in contact.
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Match the following types of friction with their characteristics:
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Which statement about the direction of limiting friction is true?
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The coefficient of kinetic friction is typically greater than that of static friction.
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Explain how surface polish affects limiting friction.
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What is the coefficient of static friction between the block and the surface if the frictional force acting on the block is 5 N and the mass of the block is 1 kg?
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A body weighing 20 kg sliding down an inclined plane has a coefficient of friction greater than 0.5.
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Calculate the force of friction acting on a scooter that weighs 120 kg when the coefficient of friction is 0.4.
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The formula showing the shortest stopping distance for an automobile on a horizontal road is ______.
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If a motor car reduces its speed from 7 m/s to a stop in 10 m, what is the total resistance to motion when brakes are applied?
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What is the distance traveled by a body before coming to rest if it is moving with a speed of 10 m/s and the coefficient of friction between the ground and the body is 0.4?
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Match the following coefficients of friction with their definitions:
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What is the coefficient of friction for a body that rises 5 m in every 13 m along a rough inclined plane?
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Study Notes
Coefficient of Friction
- Coefficient of kinetic friction ((\mu_k)) is the ratio of kinetic friction force ((f_k)) to the normal reaction force ((R)).
- For a body on a horizontal surface, normal reaction ((R)) equals the weight ((mg)).
- Static friction ((f_s)) can be expressed as (f_s \leq f_{s_{max}} = \mu_s mg).
- Kinetic friction ((f_k)) is always less than maximum static friction ((f_k < f_{s_{max}})).
- The angle of friction ((\theta)) relates to static friction by (\mu_s = \tan \theta), while the angle of repose ((\phi)) also satisfies (\mu_s = \tan \phi).
- The angle of repose is equal to the angle of friction ((\theta = \phi)).
- For a body on a rough horizontal surface experiencing retardation ((a)), the relationship is (\mu = \frac{f}{mg} = \frac{a}{g}).
Coefficient of Rolling Friction
- Coefficient of rolling friction ((\mu_r)) is defined such that rolling friction ((f_r)) is expressed as (f_r = \mu_r \cdot R).
- Relationship: (\mu_r < \mu_k < \mu_s).
Units
- Force of friction ((f)) and normal reaction ((R)) are measured in Newtons (N).
- The coefficient of friction ((\mu)) is a dimensionless quantity (no units).
Laws of Limiting Friction
- Limiting friction depends on the nature and polish of contacting surfaces.
- It acts tangentially to the surfaces in contact, opposing the direction of motion.
- Its value is independent of the contact area as long as normal reaction remains constant.
- It is directly proportional to the normal reaction ((f_{s_{max}} \propto R)), leading to the equation (f_{s_{max}} = \mu_s R).
Methods of Changing Friction
- Friction can be altered by:
- Polishing surfaces.
- Applying lubrication.
- Streamlining shapes.
- Using ball bearings for reduced contact area.
Example Calculations and Applications
- Example of static and kinetic friction calculations with given weights and coefficients.
- Applications of friction in real-world scenarios, such as automotive braking and truck acceleration can provide insights into calculating forces and coefficients.
Key Formulas
- Kinetic friction: (f_k = \mu_k \cdot mg)
- Static friction: (f_s \leq f_{s_{max}} = \mu_s mg)
- Angle of friction and repose: (\mu_s = \tan \theta = \tan \phi)
- Shortest stopping distance under constant friction: (\text{Distance} = \frac{v^2}{2\mu g})
Notable Examples
- Block on a truck with a coefficient of static friction calculated at 0.6, resulting in a frictional force of 5 N.
- Calculation of the coefficient of friction when an object slides down an inclined plane (0.4167).
- Real-world scenarios such as braking of a vehicle to illustrate friction in action.
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Description
This quiz covers the principles of friction, including kinetic and static friction equations. Understand how normal force and angle of friction relate to force in various scenarios. Test your knowledge on the concepts of friction relevant to bodies on horizontal surfaces.
