MEPX
Chapter 4 of 10All chapters
  1. 1Describing motion
  2. 2The laws of motion
  3. 3Free body diagrams
  4. 4Friction and normal forces
  5. 5Equilibrium and torque
  6. 6Work and energy
  7. 7Momentum and collisions
  8. 8Circular motion
  9. 9Projectiles
  10. 10Solving problems

Chapter 4 of 10

Friction and normal forces

The contact forces.

How friction behaves

Friction opposes relative sliding and is proportional to the normal force pressing surfaces together. Static friction can be larger than kinetic friction, which is why things jerk into motion.

  • Contact area barely matters, which surprises most people.
  • On a slope, the normal force is smaller than the weight.

Useful and unwanted

Friction is what makes walking, braking and gripping possible, and what wastes energy in every machine. Design chooses which one it wants where.