Friction: Force and Motion II – Applications

Introduction to Friction

Friction is a fundamental concept in physics that describes the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. It plays a crucial role in everyday phenomena and engineering applications.

• Friction is a force that opposes relative motion between two objects in contact.

• It acts parallel to the contact surface and always in a direction that opposes motion or attempted motion.

• Friction arises due to the microscopic roughness of surfaces and the attractive forces between molecules of the materials in contact.

Types of Friction

There are two main types of friction encountered in physics:

• Kinetic Friction: Occurs when two surfaces are in contact and moving relative to each other.

• Static Friction: Acts when surfaces are stationary relative to each other, preventing the onset of motion.

• The magnitudes of static and kinetic friction are generally different for the same pair of surfaces.

Origin and Nature of Friction

Friction is not only due to surface roughness but also to the attractive forces between the molecules of the two materials in contact. Even highly polished surfaces are not completely free of friction.

• When the normal force (the force pressing the surfaces together) increases, the frictional force also increases.

• At nonzero speeds, kinetic friction is nearly independent of speed for most materials.

Static Friction

Static friction prevents the initiation of motion between two surfaces. Its magnitude can vary up to a maximum value, depending on the normal force and the nature of the surfaces.

• The maximum static friction force is given by:

• Where is the coefficient of static friction and is the normal force.

• Static friction adjusts up to its maximum value to prevent motion.

Example: Static Friction Calculation

• A 100 N box rests on a floor with . A horizontal force of 30 N is applied via a rope. The maximum static friction is N, so the box does not move.

Kinetic Friction

Kinetic friction acts when two surfaces are sliding past each other. Its magnitude is generally less than the maximum static friction for the same surfaces.

• The force of kinetic friction is given by:

• Where is the coefficient of kinetic friction and is the normal force.

• Kinetic friction remains approximately constant for a wide range of speeds.

Example: Sliding with Kinetic Friction

• A penguin slides down an incline of 10.0° with a mass of 32.0 kg and . The kinetic friction force is .

• To find the acceleration, use Newton's second law, accounting for gravity, normal force, and friction.

Direction of Frictional Forces

The direction of the frictional force is always opposite to the direction of actual or attempted motion.

• For an object at rest, static friction acts to oppose any applied force attempting to move it.

• For a moving object, kinetic friction acts opposite to the direction of motion.

Factors Affecting Friction

• The normal force between the surfaces: Increasing the normal force increases friction.

• The nature of the surfaces: Different materials have different coefficients of friction.

• Surface area does not significantly affect friction for most solid objects.

Coefficients of Friction

The coefficients of friction are dimensionless numbers that characterize the interaction between two surfaces.

Additional info: Typical values for and range from 0.1 (ice) to 1.0 (rubber on dry concrete).

Sample Problems and Applications

• Pinned Against a Wall: A block is held against a vertical wall by a horizontal force. The frictional force opposes gravity, and the maximum static friction determines whether the block will slide.

• Trapped Hiker: A hiker uses a rope and a rock to prevent sliding. The maximum static friction between the rock and the ground determines the safety of the setup.

• Toy Chest: A child pulls a toy chest with a rope. The force required to start moving the chest is determined by the maximum static friction.

• Pushing a Sled: The maximum force that can be applied to a sled before a person sitting on it starts to slide depends on the static friction between the person and the sled.

Quick Conceptual Questions

• Does friction cause an object to speed up? Friction always acts to oppose relative motion; it cannot cause an object to speed up on its own.

• Effect of Normal Force: Increasing the normal force increases the frictional force, and vice versa.

• Direction of Friction: The direction of the frictional force is always opposite to the direction of motion or attempted motion.