3B1 Friction

Question 1

Define:

Normal force

Answer 1

Perpendicular force exerted by a surface on an object in contact with it.

This force balances the component of the object’s weight perpendicular to the surface.

Question 2

Define:

Friction

Answer 2

Force that opposes the relative motion or tendency of motion between two surfaces in contact.

Frictional forces can be static (preventing motion) or kinetic (resisting motion already occurring).

Question 3

True or False:

Frictional force always acts parallel to the surface of contact.

Answer 3

True

Frictional forces arise due to surface irregularities and interactions at a microscopic level.

Question 4

Fill in the blanks:

The magnitude of friction depends on the ______ and the ______ of the surfaces in contact.

Answer 4

normal force; coefficient of friction

The coefficient of friction quantifies the frictional interaction between surfaces.

Question 5

What causes friction?

Answer 5

It is caused by the interaction of microscopic bumps on the surface of objects.

Surface roughness and material properties determine the magnitude of friction.

Question 6

Why is friction necessary in everyday life?

Answer 6

Friction allows us to walk, drive, and hold objects without slipping.

While beneficial in some contexts, friction also causes wear and energy loss in mechanical systems.

Question 7

What is the formula for calculating the normal force of an object resting on a horizontal surface?

Answer 7

FN =mg

In the equation, m is mass and g is gravitational acceleration. On inclined planes, the normal force changes based on the angle of inclination.

Question 8

How does the normal force change on an inclined plane?

Answer 8

The normal force is reduced to FN=mgcosθ, where θ is the angle of inclination.

The component mgcosθ acts perpendicular to the inclined surface.

Question 9

True or False:

Friction can occur without a normal force.

Answer 9

False

The normal force is essential for generating frictional forces.

Question 10

What is the effect of an inclined plane on frictional force?

Answer 10

Friction decreases because the normal force is reduced.

The component of weight parallel to the plane increases, aiding motion.

Question 11

Define:

Angle of repose

Answer 11

The steepest angle of an inclined plane at which an object remains stationary due to friction.

The angle of repose is used in studying the stability of granular materials.

Question 12

True or False:

The normal force is always equal to the weight of the object.

Answer 12

False

The normal force depends on the orientation of the surface and additional forces acting on the object.

Question 13

What role does the normal force play in friction?

Answer 13

The normal force determines the maximum possible frictional force.

Frictional force is proportional to the normal force via the coefficient of friction.

Question 14

What happens to friction if the normal force increases?

Answer 14

Friction increases proportionally to the normal force.

This relationship explains why heavier objects experience more friction.

Question 15

Fill in the blank:

Frictional force is calculated as F=μ___.

Answer 15

N (normal force)

μ is the coefficient of friction, which varies between static and kinetic friction.

Question 16

Why do ice and smooth surfaces have low friction?

Answer 16

They have small coefficients of friction due to minimal surface irregularities.

This property makes ice ideal for activities like skating but hazardous for walking.

Question 17

Fill in the blank:

Friction is a force of _____ against two objects sliding against each other.

Answer 17

resistance

Friction opposes the relative motion or tendency of motion between two surfaces in contact.

Question 18

Why does friction cause wear and tear in mechanical systems?

Answer 18

It generates heat and stresses between moving parts, leading to material degradation over time.

Regular maintenance and lubrication help reduce wear.

Question 19

How does frictional force play a role in braking systems?

Answer 19

Friction between brake pads and the wheel slows down or stops a vehicle.

Brake systems are designed to maximize friction for effective stopping power.

Question 20

How does friction affect the motion of a ball in sports?

Answer 20

Friction between the ball and the surface slows its motion, allowing for control in games like soccer or basketball.

Players often exploit friction to dribble, stop, or spin the ball effectively.

Question 21

What is the difference between static and kinetic friction?

Answer 21

Static friction prevents motion, while kinetic friction opposes motion already occurring.

Static friction is generally greater than kinetic friction for the same surfaces. This means that it usually takes more force to start moving an object than to keep it moving.

Question 22

What happens when the applied force exceeds the frictional force?

Answer 22

The object will move.

This applies to both static and kinetic friction scenarios.

Question 23

What does the graph of applied force against frictional force illustrate?

Answer 23

The regions of static and kinetic friction.

Source: Wikimedia Commons

The threshold of motion is the point where an object starts moving.

Question 24

True or False:

Friction is independent of the contact area between surfaces.

Answer 24

True

Friction depends on the normal force and the coefficient of friction, not the contact area.

Question 25

# Define: Coefficient of friction

Answer 25

Number that **quantifies the ratio of the frictional force between two objects to the normal force** pressing them together. ## Footnote The coefficient of friction varies between materials and can be static or kinetic depending on whether the surfaces are stationary or in motion.

Question 26

What are the typical **ranges** of **coefficients of friction**?

Answer 26

They **range from 0 (no friction) to over 1 (high friction)**, depending on the materials and surface conditions. ## Footnote For example, ice on steel has a low coefficient (around 0.05), while rubber on dry asphalt can exceed 1.

Question 27

What is the **coefficient of static friction**?

Answer 27

It is the **ratio of the maximum static frictional force** to the normal force. ## Footnote The coefficient of static friction determines the threshold to initiate motion.

Question 28

# Fill in the blank: The **coefficient of friction** is a \_\_\_\_\_\_ quantity.

Answer 28

dimensionless ## Footnote This is because both the frictional force and normal force are measured in Newtons.

Question 29

What happens if the **coefficient of friction** is **zero**?

Answer 29

There would be no frictional force between the surfaces. ## Footnote This is an **idealized situation**, as all real surfaces have some friction

Question 30

# Fill in the blank: The **coefficient of kinetic friction** is \_\_\_\_\_\_ than the **coefficient of static friction**.

Answer 30

smaller ## Footnote Kinetic friction is lower because surfaces are already in motion.

Question 31

How do you measure the coefficient of friction **experimentally**?

Answer 31

By determining the **ratio** of frictional force to the normal force. ## Footnote Inclined planes and force sensors are commonly used for such experiments.

Question 32

How does **lubrication** affect the coefficient of friction?

Answer 32

It reduces the coefficient of friction by **minimizing surface contact**. ## Footnote Lubricants are critical in mechanical and industrial applications.

Question 33

Why does **friction** make it difficult to move heavy furniture?

Answer 33

The coefficient of static friction and normal force are **high**. ## Footnote Using rollers or sliders reduces the effective coefficient of friction.

Question 34

# Fill in the blank: Higher **coefficients of friction** result in \_\_\_\_\_\_ resistance to sliding.

Answer 34

greater ## Footnote Materials like rubber on asphalt exhibit high coefficients of friction, enhancing grip.

Question 35

Why is the **coefficient of static friction** crucial in load transportation?

Answer 35

It **determines the force needed** to initiate movement, ensuring loads remain stable during transportation. ## Footnote Proper material choices reduce sliding accidents by optimizing static friction.

Question 36

Why is the **coefficient of friction** important in designing **vehicle tires**?

Answer 36

It ensures **sufficient grip and traction** by optimizing the interaction between the tires and the road surface. ## Footnote Tire tread patterns are designed to maximize the effective coefficient of friction under various conditions.

Question 37

A sled is pulled across a horizontal surface with a **normal force of 100 N** and a **frictional force of 25 N**. What is the **coefficient of kinetic friction**?

Answer 37

μk=Ff/FN = 25/100 =0.25 ## Footnote The ratio of frictional force to normal force determines the coefficient.

Question 38

A car skids to a stop on a wet road with a **coefficient of kinetic friction of 0.4**. If the **car's weight is 12,000 N**, what is the frictional force?

Answer 38

Ff =(μk)⋅(FN) =(0.4)⋅(12,000N)=4,800N ## Footnote The frictional force is directly proportional to the coefficient of friction and the normal force.

Question 39

Explain how **frictional forces** impact **athletic performance**.

Answer 39

Friction affects **grip and stability** in sports, with higher friction aiding traction and lower friction enabling smoother motion, such as in skating. ## Footnote Equipment like shoes and surfaces are tailored to optimize friction for specific sports.