MEPX
Chapter 4 of 13All chapters
  1. 1What physics is
  2. 2Units and measurement
  3. 3Describing motion
  4. 4The laws of motion
  5. 5Forces
  6. 6Energy and work
  7. 7Momentum and collisions
  8. 8Waves
  9. 9Sound
  10. 10Light and optics
  11. 11Electricity and magnetism
  12. 12Heat and thermodynamics
  13. 13Modern physics

Chapter 4 of 13

The laws of motion

Newton in three lines.

The three

First: an object keeps doing what it is doing unless a force acts. Second: force equals mass times acceleration. Third: every force has an equal and opposite partner.

  • The first law is why you lurch forward when a bus brakes.
  • The third law pairs act on different objects, which is why they do not cancel out.

Using the second law

Add up the forces on an object, then divide by its mass to get the acceleration. Most problems are that one step plus careful bookkeeping about direction.