Inertia and Newton's Third Law

• Inertia


  The tendency for motion to remain unchanged
• Inertia


  1. Until acted upon by a resultant force, objects at rest stay at rest and objects moving at a steady speed will stay moving at that speed (Newton's First Law)

  2. An object's inertial mass measures how difficult it is to change the velocity of an object

  3. Inertial mass can be found using Newton's Second Law of F=ma (see the last page)
• Newton's Third Law


  When two objects interact, the forces they exert on each other are equal and opposite
• Newton's Third Law


  • If you push something, say a shopping trolley, the trolley will push back against you, just as hard
  • As soon as you stop pushing, so does the trolley
• If the forces are always equal, how does anything ever go anywhere?
• The important thing to remember is that the two forces are acting on different objects
• Newton's Third Law in an equilibrium situation

  • A man pushing against a wall
  • A book resting on a table
• The pairs of forces due to Newton's Third Law in the case of a book resting on a table are: 1) The weight of book is pulled down by gravity from Earth (W) and the book also pulls back up on the Earth (W) 2) The normal contact force from the table pushing up on the book (N) and the normal contact force from the book pushing down on the table (N)
• These forces in the case of a book resting on a table are not Newton's Third Law, as they are different types of forces acting on the book