Friction | Jamb(UTME)

Jamb(utme) key points on static and dynamic friction; coefficient of limiting friction and its determination; reduction of friction

Static and Dynamic Friction

1. Friction is a force that opposes the relative motion of two surfaces in contact.
2. Static friction acts when an object is at rest and prevents it from moving.
3. Static friction increases with the applied force until it reaches its maximum limit.
4. The maximum force of static friction is called the limiting friction.
5. Once the applied force exceeds the limiting friction, the object starts moving.
6. Dynamic friction (or kinetic friction) acts when an object is in motion.
7. Dynamic friction is usually smaller than the limiting static friction.
8. Static friction prevents motion, while dynamic friction resists motion.
9. Static friction acts up to a limit, whereas dynamic friction remains constant during motion.
10. Examples of static friction include pushing a heavy box that initially doesn't move.
11. Examples of dynamic friction include sliding a book across a table.
12. Friction depends on the nature of the surfaces in contact (e.g., rough or smooth).
13. Friction is independent of the area of contact between two surfaces.
14. Static and dynamic friction are proportional to the normal force acting between the surfaces.
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Coefficient of Limiting Friction

15. The coefficient of limiting friction $(\mu_s)$ is a ratio of the limiting friction force to the normal force.
16. The formula is ${\mu}_s = \frac{F_{limiting}}{F_{normal}}$.
17. $F_{limiting}$ is the maximum force before the object starts to move.
18. $F_{normal}$ is the perpendicular force exerted by the surface on the object.
19. The coefficient of limiting friction has no unit since it is a ratio.
20. It depends on the materials of the surfaces in contact.
21. A high $\mu_s$ means the surfaces have high friction (e.g., rubber on asphalt).
22. A low $\mu_s$ means the surfaces have low friction (e.g., ice on metal).
23. The coefficient of limiting friction is generally larger than the coefficient of dynamic friction.
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Determination of the Coefficient of Limiting Friction

24. To determine $\mu_s$, use a flat surface and an object whose weight is known.
25. Gradually apply force to the object until it starts to move.
26. Measure the force at the exact moment the object begins to move—this is $F_{limiting}$.
27. Measure the normal force, which is equal to the object's weight if the surface is horizontal.
28. Divide $F_{limiting}$ by $F_{normal}$ to find $\mu_s$.
29. A common apparatus for this experiment includes a spring balance to measure force.
30. Inclined planes can also be used by gradually tilting the surface until the object slides.
31. On an inclined plane, $\mu_s = \tan\theta$, where $\theta$ is the angle of inclination when sliding begins.
32. Repeated trials improve accuracy by averaging the results.
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Reduction of Friction

33. Friction can be reduced to improve efficiency in machines and reduce wear.
34. Lubricants, such as oil or grease, are applied between surfaces to reduce friction.
35. Lubricants form a thin layer that minimizes direct contact between surfaces.
36. Ball bearings reduce friction by replacing sliding motion with rolling motion.
37. Polishing surfaces makes them smoother, reducing friction.
38. Streamlining reduces air resistance, a type of fluid friction.
39. Using soft materials, like rubber, on hard surfaces can sometimes reduce friction.
40. Friction in fluids is reduced by adding substances like detergents or surfactants.
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Importance of Friction

41. Friction is essential for walking; without it, we would slip.
42. It allows cars to move by providing grip between tires and the road.
43. Friction is needed to stop moving objects, such as braking a car.
44. It helps in holding objects, like a nail in wood or screws in metal.
45. Excessive friction causes wear and tear in machines, increasing maintenance costs.
46. Reducing friction is essential in machines to improve energy efficiency.
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Factors Affecting Friction

47. The roughness or smoothness of the surfaces in contact affects friction.
48. The normal force pressing the two surfaces together affects friction directly.
49. Friction does not depend on the surface area in contact.
50. The type of material and whether lubricants are used influence the amount of friction.
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Jamb(utme) key points on advantages and disadvantages of friction; qualitative treatment of viscosity and terminal velocity; Stoke's law

Advantages and Disadvantages of Friction

1. Friction allows us to walk without slipping.
2. It helps in gripping and holding objects.
3. Friction enables cars to move and stop by providing traction to tires.
4. It is essential for writing, as it allows pens and pencils to leave marks on paper.
5. Machines rely on friction for belts and pulleys to transmit motion.
6. Friction produces heat, which is useful in tasks like lighting matches.
7. Excessive friction causes wear and tear on surfaces, reducing their lifespan.
8. Friction increases energy consumption in machines by resisting motion.
9. It can cause overheating in engines, reducing efficiency.
10. Lubrication is used to minimize the negative effects of friction in machinery.
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Qualitative Treatment of Viscosity

11. Viscosity is the resistance of a fluid to flow.
12. It arises due to the internal friction between layers of fluid.
13. Highly viscous fluids flow slowly (e.g., honey), while low-viscosity fluids flow quickly (e.g., water).
14. Viscosity depends on the fluid's temperature and composition.
15. As temperature increases, viscosity decreases for liquids but increases for gases.
16. Viscosity plays a role in processes like lubrication and blood flow.
17. Thick fluids with high viscosity are harder to stir or pour than thin fluids.
18. The unit of viscosity in the SI system is pascal-second (Pa·s).
19. Everyday examples include motor oil, which has different viscosities for summer and winter use.
20. A fluid with no viscosity (ideal fluid) does not exist in reality.
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Factors That Affect Viscosity

21. Temperature: Increasing temperature lowers the viscosity of liquids but increases the viscosity of gases.
22. Composition: Heavier molecules or complex structures increase viscosity.
23. Pressure: Higher pressure can increase viscosity in certain cases.
24. Impurities: Dissolved substances can either increase or decrease a fluid's viscosity.
25. Intermolecular forces: Stronger interactions between molecules lead to higher viscosity.
26. Viscosity varies across different substances (e.g., water is less viscous than syrup).
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Terminal Velocity

27. Terminal velocity is the constant speed that a falling object reaches when the forces of gravity and air resistance are balanced.
28. At terminal velocity, the net force on the object is zero, and it stops accelerating.
29. Heavier objects generally have higher terminal velocities than lighter ones.
30. Streamlined shapes help reduce air resistance, increasing terminal velocity.
31. Skydivers reach terminal velocity before deploying their parachutes.
32. Terminal velocity depends on the object’s size, shape, and the medium it moves through.
33. The formula for terminal velocity in a viscous medium is derived using Stoke's law.
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Stoke's Law

34. Stoke's law explains the force of viscous drag on small spherical objects moving through a fluid.
35. The viscous drag force $(F)$ is given by $F = 6\pi \eta r v$, where:
    • $\eta$ is the fluid’s viscosity,
    • $r$ is the radius of the sphere,
    • $v$ is the object’s velocity.
36. Stoke's law applies to smooth, spherical objects moving at low speeds in a viscous fluid.
37. It is used to calculate the terminal velocity of objects in fluids.
38. Small particles, like raindrops or pollen grains, behave predictably under Stoke’s law.
39. It is critical in industries like paint, where viscosity affects flow and coverage.
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Analysis of Terminal Velocity and Viscosity

40. Terminal velocity is reached when the drag force equals the gravitational force on an object.
41. Higher viscosity fluids produce lower terminal velocities for objects.
42. Large or dense objects take longer to reach terminal velocity than small ones.
43. Objects with larger surface areas experience more drag, reducing terminal velocity.
44. In free fall, air resistance acts upward while gravity acts downward, creating terminal velocity.
45. Terminal velocity can be increased by making objects streamlined to reduce drag.
46. In fluids, terminal velocity depends on both viscosity and the radius of the object.
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Applications of Viscosity and Terminal Velocity

47. Viscosity affects the design of pipelines, ensuring smooth flow of liquids like oil or water.
48. Terminal velocity helps design parachutes to slow down free-falling objects safely.
49. Stoke’s law is applied in sedimentation studies to separate particles of different sizes.
50. Engineers use viscosity measurements to select lubricants for machinery based on operating conditions.
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