Lecture 13 - Ground Reaction Force Flashcards

1
Q

What is friction?

A

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

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2
Q

How does friction allow movement or acceleration of the body?

A

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.

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3
Q

What is the formula for calculating the force of friction?

A

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.

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4
Q

In which direction does friction act?

A

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

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5
Q

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

A

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.

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6
Q

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

A

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

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7
Q

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

A

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

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8
Q

When is friction greatest

A

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

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9
Q

Static friction:

A

the friction between two surfaces when they are not moving

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10
Q

What is kinetic friction?

A

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

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11
Q

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

A

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.

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12
Q

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

A

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.

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13
Q

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

A

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

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14
Q

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

A

The maximum static friction force is greater than kinetic friction.

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15
Q

How does kinetic friction compare to maximum static friction?

A

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.

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16
Q

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

A

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.

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17
Q

Magnitude of Friction
Depends on four factors:

A
  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)

18
Q

What is the coefficient of friction?

A

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.

19
Q

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

A

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

20
Q

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

A

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.

21
Q

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

A

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).

22
Q

What is the formula for calculating the coefficient of friction?

A

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.

23
Q

Why does the coefficient of friction not have units?

A

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.

24
Q

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

A

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.

25
Q

What is the normal force?

A

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.

26
Q

How do you determine the magnitude of the normal force?

A

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.

27
Q

How does the normal force relate to friction?

A

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.

28
Q

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

A

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.

29
Q

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

A

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.

30
Q

What is rolling resistance?

A

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.

31
Q

How does rolling resistance differ from friction?

A

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.

32
Q

What causes rolling resistance?

A

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.

33
Q

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

A

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.

34
Q

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

A

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.

35
Q

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

A

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.

36
Q

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

A

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)

37
Q

Why does friction increase when pushing an object?

A

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.

38
Q

How does a layer of fluid between surfaces affect friction?

A

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.

39
Q

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

A
  1. Nature of the fluid (consistency; water vs. oil)
  2. Fluid temperature
  3. Velocity between the two surfaces
40
Q

What are ground reaction forces?

A

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.