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Difference Between Static and Dynamic Friction

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What is Static and Dynamic Friction: Introduction

To differentiate between static and dynamic friction: Static and dynamic friction are two types of forces that arise when two surfaces are in contact with each other. Static friction refers to the force that prevents an object from moving when a force is applied to it but is not sufficient to overcome the resistance. It acts in the opposite direction to the applied force and adjusts to match its magnitude until the object starts moving. Dynamic friction, on the other hand, comes into play once the object is in motion. It is the force that opposes the motion of the object as it slides or moves over a surface. The magnitude of dynamic friction is typically slightly lower than static friction. Read further for more detail. 


Defining Static Friction

Static friction is a type of force that exists between two surfaces in contact when an external force is applied to one of the surfaces, but the objects remain at rest. It acts in the direction opposite to the applied force and prevents the relative motion between the surfaces. The magnitude of static friction is determined by the force applied and the nature of the surfaces in contact. It adjusts itself to match the applied force until the force reaches a critical value called the maximum static friction. Once this threshold is exceeded, the static friction is overcome, and the objects start to move, transitioning into dynamic friction. The features of static friction are: 


  • Direction: Static friction always acts in the opposite direction to the applied force, preventing the object from moving.

  • Variable Magnitude: The magnitude of static friction adjusts itself to match the applied force until it reaches a maximum value known as the maximum static friction. 

  • Threshold: Static friction prevents the object from moving as long as the applied force does not exceed the maximum static friction. Once the force surpasses this threshold, the object will start to move.

  • Dependent on Surface Properties: The coefficient of static friction is a measure of the roughness or smoothness of the surfaces in contact. It determines the magnitude of static friction and can vary depending on the materials involved.

  • Essential for Stability: Static friction plays a crucial role in maintaining the stability of objects at rest. It allows objects to remain stationary on inclined planes, prevents slipping or sliding, and aids in gripping or traction between surfaces.


Defining Dynamic Friction

Dynamic friction, also known as kinetic friction, refers to the force that opposes the motion of an object as it slides or moves across a surface. Unlike static friction, which acts when the object is at rest, dynamic friction comes into play once the object is in motion. The magnitude of dynamic friction is generally slightly lower than static friction. It depends on the nature of the surfaces in contact and the normal force between them. The coefficient of dynamic friction is a measure of the roughness or smoothness of the surfaces and determines the strength of the frictional force. The features of dynamic friction are:


  • Direction: Dynamic friction always acts in the direction opposite to the motion of the object. It opposes the relative motion between the surfaces in contact.

  • Magnitude: The magnitude of dynamic friction is typically slightly lower than static friction. It depends on factors such as the nature of the surfaces, the normal force, and the coefficient of dynamic friction.

  • Independent of Velocity: Unlike other forms of resistance, such as air resistance, dynamic friction is generally independent of the velocity of the object. The force remains relatively constant once the object is in motion, as long as the surfaces and conditions remain unchanged.

  • Wear and Heat Generation: Dynamic friction generates heat and can cause wear and tear on the surfaces in contact. This effect is particularly noticeable when moving at higher speeds or with rough surfaces.

  • Decreases with Lubrication: The application of lubricants or reducing agents between the surfaces can significantly reduce dynamic friction. Lubrication helps to separate the surfaces and minimize the resistance to motion.


Static and Dynamic Friction Differences

S. No

Category 

Static Friction

Dynamic Friction

1

Definition

Friction when an object is at rest

Friction when an object is in motion

2

Initiation of motion

Prevents motion until a threshold is exceeded

Opposes the motion once the object is in motion

3

Magnitude

Adjusts to match the applied force until the maximum static friction is reached

Slightly lower than static friction

4

Direction

Acts in the opposite direction to the applied force

Acts in the opposite direction to the object's motion

5

Dependent on

Surface roughness, normal force, and coefficient of static friction

Surface roughness, normal force, and coefficient of dynamic friction

Velocity dependency

Independent of velocity

Generally independent of velocity


This tabular comparison highlights the key differences between static and dynamic friction, including their definitions, initiation of motion, magnitude, direction, dependency on factors, and velocity dependency.


Summary 

Static friction is the frictional force that prevents the motion of an object at rest, while dynamic friction is the frictional force that opposes the motion of an object that is sliding or moving. Static friction acts between two surfaces that are not moving relative to each other, while dynamic friction acts between surfaces in relative motion. The magnitude of static friction increases with the applied force until it reaches a maximum value, while dynamic friction is generally lower and remains relatively constant as long as the relative motion continues at a constant speed.

Last updated date: 26th Sep 2023
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FAQs on Difference Between Static and Dynamic Friction

1. What causes static friction to occur?

Static friction occurs due to the interlocking irregularities and molecular interactions between the surfaces in contact. It arises when an external force is applied to an object at rest, creating resistance that prevents the object from moving. The roughness of the surfaces, the applied force, and the normal force between the surfaces determine the magnitude of static friction. It is the result of the microscopic interactions between atoms and molecules at the contact interface, creating a force that opposes the impending motion.

2. Does the magnitude of dynamic friction change with the speed of the object?

The magnitude of dynamic friction generally remains relatively constant with the speed of the object. Once the object is in motion, dynamic friction opposes its motion with a force that depends on the nature of the surfaces and the normal force between them. While the speed of the object can affect factors such as the generation of heat due to friction, the overall magnitude of the dynamic friction force does not significantly change as long as the relative motion between the surfaces continues at a constant speed.

3. How does static friction prevent an object from moving?

Static friction prevents an object from moving by exerting an opposing force in response to an applied force trying to set the object in motion. When an external force is applied to an object at rest, the static friction force increases proportionally until it reaches a maximum value. This maximum static friction force matches the applied force, effectively balancing it and preventing the object from moving. The surfaces in contact have interlocking irregularities that create resistance and require an adequate force to overcome, thereby maintaining the object's stationary position.

4. How does the normal force influence static and dynamic friction?

The normal force, which is the perpendicular force exerted by a surface on an object, plays a significant role in determining the magnitude of both static and dynamic friction. The normal force affects the amount of intermolecular contact between the surfaces in contact. As the normal force increases, the amount of intermolecular contact increases, resulting in a stronger frictional force. Therefore, a higher normal force leads to an increase in both static and dynamic friction. 

5. What happens to dynamic friction when surfaces are lubricated?

When surfaces are lubricated, the presence of a lubricant reduces the friction between them, including dynamic friction. The lubricant forms a protective layer between the surfaces, minimizing direct contact and reducing frictional resistance. This smoother interface allows the objects to move more freely with less resistance. Lubrication helps to decrease the magnitude of dynamic friction, making it easier for objects to slide or move smoothly over each other, improving efficiency and reducing wear and tear on surfaces.