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Concept of Friction is taught in the note for Friction. Read it below and look for the section on this topic.

Friction

11 min read

Concept of friction

Friction is a force that opposes the motion of one surface over another. It acts along the surfaces, opposite to the direction the object moves or tends to move. It is caused by the roughness of surfaces: tiny bumps lock into each other, and molecules attract. Friction produces heat and wear.

Types of friction

TypeWhen it actsExample
Static frictionBetween surfaces at rest; adjusts up to a maximum (limiting friction)A heavy box that does not move when you push gently
Kinetic (sliding) frictionBetween surfaces sliding over each other; usually less than the limiting static frictionBrakes on a wheel, a sliding box
Rolling frictionWhen an object rolls; much smaller than slidingWheels, ball bearings
Fluid friction (drag)In liquids and gases; increases with speedParachute, swimming, car at high speed
Static and kinetic frictionA graph of friction force against the applied force on a block. Friction matches the applied force and rises in a straight line until it reaches the limiting friction, the largest static friction. Once the block moves, friction drops to a smaller constant value called kinetic or sliding friction.applied forcefriction forcelimiting frictionstatic: friction = applied forcekinetic (sliding) friction: constantblock starts to move

Laws of friction

LawStatement
1Friction opposes motion and acts parallel to the surfaces in contact
2The limiting friction is proportional to the normal reaction between the surfaces
3Friction does not depend on the area of contact (for hard dry surfaces)
4Kinetic friction is nearly independent of speed and is less than the limiting static friction
F = friction force, R = normal reaction, μ = coefficient of friction (no unit). On a flat surface R = mg.
  • coefficient of friction
  • normal reaction (N)
Forces on a blockA block resting on a table and being pulled to the right. Weight acts downwards, the normal reaction from the table acts upwards, the pull acts to the right and friction acts to the left.blockR normal reactionW weightP pullfriction F
Try itFriction on a block

Choose the mass of a block and the coefficient of friction. See the weight and the force needed to keep it sliding at constant speed on a flat surface. (g = 10 N/kg)

Worked examplePushing a crate

A crate of mass 40 kg is pushed along a floor at constant speed. The coefficient of kinetic friction is 0.3. What force is needed? (g = 10 N/kg)

  1. At constant speed the push equals the friction.
  2. R = mg = 40 × 10 = 400 N.

Answer: A force of 120 N is needed.

Advantages and disadvantages; reducing friction

Useful frictionHarmful friction
Walking and grippingWear of machine parts
Brakes and tyresHeat wasted in engines
Nails and screws holdMakes it harder to move heavy loads

Friction is reduced by lubricants (oil, grease, graphite), ball bearings and rollers, streamlining, and by polishing surfaces. It is increased by roughening surfaces (tyre treads, grooves in shoes) and by sprinkling sand on icy roads.

Check yourself The coefficient of friction μ is defined as…
Check yourself Which will reduce friction between moving parts?
Check yourself A box of weight 200 N slides at constant speed under a 50 N push. What is μ?

Key terms

Friction
A force that opposes the relative motion of two surfaces in contact.
Limiting friction
The maximum static friction before sliding starts.
Coefficient of friction
The ratio of friction to normal reaction.
Lubricant
A substance that reduces friction.

Summary

  • Friction opposes motion; static, kinetic, rolling and fluid friction.
  • F = μR; it does not depend on the area of contact.
  • Friction is useful for grip and braking but wastes energy and causes wear.
  • Reduce friction with lubricants, bearings and streamlining.
Types of Friction