Lecture 13 - Ground Reaction Force

Question 1

What is friction?

Answer 1

Friction is a force that develops between two surfaces, preventing them from slipping past each other.

Question 2

How does friction allow movement or acceleration of the body?

Answer 2

Friction allows the ground to exert a significant enough force on the foot to accelerate the body, as it acts against the direction of slipping.

Question 3

What is the formula for calculating the force of friction?

Answer 3

The force of friction can be calculated using the formula F=μR, where

F is the frictional force,

μ is the coefficient of friction, and

R is the normal force, which acts perpendicular to the surfaces in contact.

Question 4

In which direction does friction act?

Answer 4

Friction acts in the direction opposite to the direction of motion or impending motion.

Question 5

How does the magnitude of friction affect the motion between two objects?

Answer 5

The magnitude of friction determines how easy or difficult it is to create a sliding motion between two objects. Greater friction means increased difficulty for the objects to slip with respect to each other.

Question 6

What happens to the difficulty of sliding motion between two objects if the friction between them is increased?

Answer 6

If the friction between two objects is increased, the difficulty of creating a sliding motion between them also increases.

Question 7

Does the area of contact between two surfaces affect the frictional force between them?

Answer 7

No, the frictional force does not increase with greater surface area since the area isn’t part of the friction equation.

Question 8

When is friction greatest

Answer 8

Friction is greatest at (C), just before the object moves

Question 9

Static friction:

Answer 9

the friction between two surfaces when they are not moving

Question 10

What is kinetic friction?

Answer 10

Kinetic friction, also known as dynamic friction, is the friction that occurs between surfaces in motion relative to each other.

Question 11

How does the magnitude of kinetic friction change with the speed of the moving surfaces or the force applied to them?

Answer 11

The magnitude of kinetic friction remains constant, regardless of how fast the two surfaces are moving or how hard you push them against each other.

Question 12

Under what condition will sliding between two surfaces come to a stop?

Answer 12

Sliding will continue to occur between the two surfaces until the applied force drops below the level of kinetic friction, causing them to slow down and eventually come to a stop.

Question 13

How does the force of static friction compare to the force applied on an object before it starts moving?

Answer 13

The force of static friction is proportional to and equal to the applied force up until the point where movement begins.

Question 14

Is the maximum static friction force greater or less than kinetic friction?

Answer 14

The maximum static friction force is greater than kinetic friction.

Question 15

How does kinetic friction compare to maximum static friction?

Answer 15

Kinetic friction is constant and less than the maximum static friction.

This means that the applied force required to maintain motion is greater than the kinetic friction force but less than the maximum static friction.

Question 16

Why is the frictional force during motion (kinetic friction) considered constant?

Answer 16

Kinetic friction is considered constant because its magnitude does not depend on the speed of the moving surfaces or the magnitude of the force applied to them; it remains consistent under constant conditions of movement.

Question 17

Magnitude of Friction
Depends on four factors:

Answer 17

1. The applied force that the friction is resisting
2. Whether motion is occurring or not
3. The coefficient of friction () ; this refers to the type of surface (steel on ice vs. steel on wood)

4.The magnitude of the normal force (R)

Question 18

What is the coefficient of friction?

Answer 18

The coefficient of friction is a unitless number that indicates the ease of sliding between two surfaces.

It quantifies the frictional force resistance that one surface encounters when moving over another.

Question 19

Can the coefficient of friction be determined for a single surface?

Answer 19

No, the coefficient of friction cannot be determined for a single surface alone; it is a measure of the interaction between two surfaces.

Question 20

How does the coefficient of friction relate to the roughness and hardness of a surface?

Answer 20

The coefficient of friction is influenced by the relative roughness and hardness of the surfaces in contact, such as ice on ice versus sandpaper on sandpaper, affecting their ease of sliding over each other.

Question 21

Are there different values of the coefficient of friction for moving and motionless surfaces?

Answer 21

Yes, there are different values for the coefficient of friction depending on whether the surfaces in contact are moving (kinetic coefficient, denoted as k) or are motionless (static coefficient, denoted as s).

Question 22

What is the formula for calculating the coefficient of friction?

Answer 22

The coefficient of friction can be calculated using the formula: coefficient of friction = friction force / normal force. This formula quantifies the frictional resistance relative to the perpendicular force (normal force) pressing the surfaces together.

Question 23

Why does the coefficient of friction not have units?

Answer 23

The coefficient of friction is a ratio of two forces (friction force and normal force), both of which are measured in the same units (e.g., Newtons). Since it is a ratio of like units, the units cancel out, making the coefficient of friction a unitless value.

Question 24

How does the coefficient of friction affect the force required to initiate or maintain motion between two surfaces?

Answer 24

The coefficient of friction directly affects the force required to initiate or maintain motion between two surfaces. A higher coefficient indicates greater resistance to sliding, requiring more force to overcome friction and initiate motion, while a lower coefficient suggests easier sliding with less force required.

Question 25

What is the normal force?

Answer 25

The normal force is the force exerted by the surface below pushing up on the surface above. It acts perpendicular to the sliding surfaces and is a reaction to other forces acting on the object, primarily gravity.

Question 26

How do you determine the magnitude of the normal force?

Answer 26

The magnitude of the normal force is determined by considering all of the vertical forces acting on the object, including gravity. It is the sum of all forces acting perpendicular to the contact surfaces.

Question 27

How does the normal force relate to friction?

Answer 27

The normal force directly influences the friction force between two surfaces. Since the frictional force is proportional to the normal force, reducing the normal force will consequently reduce the friction force.

Question 28

What does it mean if the net force acting on an object is zero?

Answer 28

If the net force acting on an object is zero, it means that the object is either at rest or moving at a constant velocity, indicating that there is no acceleration. This is based on the principle that if mass is not zero, and acceleration is zero, then the net force must also be zero.

Question 29

Can you alter the normal force to control friction? If so, how?

Answer 29

Yes, you can alter the normal force to control friction. By reducing the vertical forces acting on the object (for example, by lifting part of the object to reduce the force due to gravity it experiences), the normal force is decreased, which in turn reduces the friction force between the surfaces.

Question 30

What is rolling resistance?

Answer 30

Rolling resistance is the resistance to motion that occurs between a wheel and the ground. It is not solely due to friction but is primarily due to the deformation of the wheel and the ground below it.

Question 31

How does rolling resistance differ from friction?

Answer 31

Although treated similarly to friction because it resists motion, rolling resistance differs in that it involves more than just the stickiness between two materials. It primarily results from the deformation aspects of the wheel and the surface it rolls on, rather than the direct surface-to-surface contact friction.

Question 32

What causes rolling resistance?

Answer 32

Rolling resistance is primarily caused by the deformation of the wheel and the ground below. As the wheel rolls, both the wheel and the ground deform slightly, which requires additional energy to overcome, leading to resistance against the rolling motion.

Question 33

Is the coefficient of rolling friction the same as the coefficient of sliding friction?

Answer 33

No, the coefficient of "rolling friction" is different from the coefficient of sliding friction. The term "rolling friction" refers to a value that accounts for more than just the surface stickiness, including factors like deformation of the wheel and the ground, making it distinct from sliding friction coefficients.

Question 34

How does the magnitude of rolling resistance compare to the magnitude of friction between two flat surfaces?

Answer 34

The magnitude of rolling resistance is much smaller than the magnitude of friction between two flat surfaces. This is because rolling resistance mainly involves overcoming deformation forces, which are typically less than the forces needed to overcome static or kinetic friction between flat surfaces in direct contact.

Question 35

How does pulling an object affect the friction compared to pushing it?

Answer 35

When pulling an object, friction is reduced because the vertical component of the pulling force reduces the normal force. In contrast, when pushing, friction is increased because there is no vertical component to alter the normal force, making it effectively larger.

Question 36

What happens to the normal force when you pull an object?

Answer 36

When you pull an object, the vertical component of the pulling force acts to reduce the normal force. The equation Fg = FN + Fty shows that part of the gravitational force (Fg) is balanced by the vertical component of the pulling force (Fty) thus reducing the normal force (FN)

Question 37

Why does friction increase when pushing an object?

Answer 37

Friction increases when pushing an object because there is no vertical component of the pushing force to reduce the normal force. The entire gravitational force is balanced by the normal force (F g =F N), which means the normal force remains high, increasing friction. This effect is further amplified if there is a downward or negative force applied, increasing the normal force even more.

Question 38

How does a layer of fluid between surfaces affect friction?

Answer 38

A layer of fluid between surfaces reduces friction. This occurs because the fluid layer acts as a lubricant, minimizing the direct contact between the surfaces, which in turn reduces the frictional force.

Question 39

What factors influence the magnitude of frictional force when a fluid film is present between surfaces?

Answer 39

1. Nature of the fluid (consistency; water vs. oil) 2. Fluid temperature 3. Velocity between the two surfaces

Question 40

What are ground reaction forces?

Answer 40

round reaction forces are the forces exerted by the ground on us in response to the forces we apply to it. According to Newton's 3rd law, for every action, there is an equal and opposite reaction. Thus, as we push on the ground, the ground pushes back on us in all directions.