Define static and kinetic friction.

Short Answer:

Static friction is the frictional force that acts between two surfaces that are in contact but not moving relative to each other. It prevents motion when an external force is applied. Static friction increases with the applied force until it reaches its maximum value, known as limiting friction.

Kinetic friction (also called dynamic or sliding friction) is the frictional force acting between two surfaces when one surface slides or moves over the other. Once motion starts, kinetic friction remains almost constant and is usually less than static friction.

Detailed Explanation :

Static and Kinetic Friction

In Engineering Mechanics, friction is one of the most important concepts that deals with the resistance to motion between two surfaces in contact. Depending on whether the surfaces are at rest or in motion, friction is classified into two types — static friction and kinetic friction. Understanding these two types is essential for analyzing the equilibrium and motion of bodies in practical engineering problems such as braking, rolling, and machine design.

  1. Static Friction

Static friction is the frictional force that acts on a body when it is at rest but an external force tends to move it. It resists the initial motion of the body.

Definition:
Static friction may be defined as “the frictional force that develops between two surfaces in contact when an external force is applied but motion has not yet started.”

Explanation:
When an external force is applied to a body at rest, the body does not immediately start to move. This is because a resisting force called static friction acts in the opposite direction to the applied force. As the applied force increases, the static friction also increases proportionally, up to a certain maximum value. Beyond this limit, the body begins to move.

This maximum value of static friction is known as limiting friction (Fₗ). Once the applied force exceeds this limiting value, the body starts to slide, and kinetic friction comes into play.

Mathematically,

where,

  • Fₛ = Static frictional force
  • μₛ = Coefficient of static friction
  • N = Normal reaction between surfaces

When the applied force equals μₛN, it becomes the limiting friction and the body is just about to move.

Characteristics of Static Friction:

  1. Acts only when there is no relative motion between the surfaces.
  2. Increases with the applied force until it reaches the limiting value.
  3. Direction is always opposite to the direction of the applied force.
  4. Depends on the nature and condition of the contact surfaces.
  5. Its maximum value is generally higher than kinetic friction.

Example:

  • A heavy box resting on the floor does not move when pushed gently. The resisting force keeping it stationary is static friction.
  • When trying to push a car that is not moving, the initial resistance felt is due to static friction.
  1. Kinetic Friction

Kinetic friction, also known as dynamic friction or sliding friction, is the frictional force acting between two surfaces that are already in relative motion.

Definition:
Kinetic friction may be defined as “the resisting force acting between two surfaces when one surface is sliding or moving over the other.”

Explanation:
When a body starts moving, the contact points between the two surfaces break and reform continuously. During this process, the resisting force becomes slightly lower and remains nearly constant, even if the velocity changes. This constant resisting force is known as kinetic friction.

The magnitude of kinetic friction depends mainly on the normal reaction and the coefficient of kinetic friction (μₖ). It is expressed as:

where,

  • Fₖ = Kinetic frictional force
  • μₖ = Coefficient of kinetic friction
  • N = Normal reaction between the surfaces

Since it is easier to keep a body moving than to start its motion, μₖ is always less than μₛ.

Characteristics of Kinetic Friction:

  1. Acts when two surfaces are sliding or moving over each other.
  2. Its value remains nearly constant during motion.
  3. It is always less than limiting friction.
  4. It depends on the normal reaction and the nature of the surfaces.
  5. Acts opposite to the direction of motion.

Example:

  • When you push a block across a table, the force opposing its motion after it starts sliding is kinetic friction.
  • The frictional resistance on a moving sledge or a sliding door is due to kinetic friction.

Difference Between Static and Kinetic Friction

Although both static and kinetic friction oppose motion, they differ in their conditions and characteristics.

  • Condition of Action:
    Static friction acts when the body is at rest; kinetic friction acts when the body is moving.
  • Magnitude:
    Static friction increases with the applied force up to the limiting value, while kinetic friction remains constant once motion starts.
  • Relationship with Normal Reaction:
    Both depend on the normal reaction, but the coefficients differ — μₛ > μₖ.
  • Work Done:
    Static friction does no work since there is no displacement; kinetic friction does work as motion occurs.

Graphical Representation of Static and Kinetic Friction

If we plot a graph between the applied force and the frictional force:

  • Initially, friction increases linearly with applied force (static friction region).
  • It reaches a peak (limiting friction).
  • Once motion begins, the frictional force drops slightly and remains constant (kinetic friction region).

This graph helps visualize the transition from static to kinetic friction.

Coefficient of Friction

The coefficient of static friction (μₛ) and the coefficient of kinetic friction (μₖ) are dimensionless quantities that describe the ratio of frictional force to the normal reaction.

  • Usually, μₛ > μₖ.
  • Their values depend on material type, surface finish, and lubrication.

Typical values are:

  • Steel on steel: μₛ = 0.6, μₖ = 0.4
  • Wood on wood: μₛ = 0.5, μₖ = 0.3
  • Rubber on concrete: μₛ = 0.9, μₖ = 0.8

Applications in Engineering

  1. Braking Systems:
    Static friction helps initiate braking, while kinetic friction maintains braking force.
  2. Machinery Design:
    Knowledge of friction helps in reducing wear and energy loss.
  3. Transportation:
    The friction between tires and road surface ensures vehicle stability.
  4. Power Transmission:
    Belt drives and clutches rely on friction for transmitting motion.
  5. Construction:
    Friction helps stabilize objects such as ladders or scaffolding.
Conclusion

In conclusion, static friction acts when two surfaces are in contact but not moving, and it prevents motion up to its limiting value, while kinetic friction acts when the surfaces are in relative motion and remains nearly constant. Static friction is always greater than kinetic friction. Both types play vital roles in everyday life and engineering applications by enabling control over motion, providing grip, and ensuring stability in mechanical systems.