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
| Type | When it acts | Example |
|---|---|---|
| Static friction | Between surfaces at rest; adjusts up to a maximum (limiting friction) | A heavy box that does not move when you push gently |
| Kinetic (sliding) friction | Between surfaces sliding over each other; usually less than the limiting static friction | Brakes on a wheel, a sliding box |
| Rolling friction | When an object rolls; much smaller than sliding | Wheels, ball bearings |
| Fluid friction (drag) | In liquids and gases; increases with speed | Parachute, swimming, car at high speed |
Laws of friction
| Law | Statement |
|---|---|
| 1 | Friction opposes motion and acts parallel to the surfaces in contact |
| 2 | The limiting friction is proportional to the normal reaction between the surfaces |
| 3 | Friction does not depend on the area of contact (for hard dry surfaces) |
| 4 | Kinetic friction is nearly independent of speed and is less than the limiting static friction |
- coefficient of friction
- normal reaction (N)
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)
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)
- At constant speed the push equals the friction.
- R = mg = 40 × 10 = 400 N.
Answer: A force of 120 N is needed.
Advantages and disadvantages; reducing friction
| Useful friction | Harmful friction |
|---|---|
| Walking and gripping | Wear of machine parts |
| Brakes and tyres | Heat wasted in engines |
| Nails and screws hold | Makes 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.
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.