Newton’s 2nd Law

Newton’s 2nd Law: Fnet = ma


  1. Force of GravityFg = mg = weight
  • Weight is defined as the force of gravity on an object.
  • An object’s mass (its inertia) never changes. Its weight depends on the acceleration of gravity.
  1. Free body diagrams:
  • Draw only the object and the forces acting on it.
  • Do not draw in velocity, acceleration…
  • Once the free body diagram is drawn, write an equation using Fnet = ma.
  • Example: A 20 kg cart is pulled forward by a force of 45 Newtons. There is a force of friction of 15 Newtons acting on the object.

Free body diagrams can become much more complicated, but the basic procedure is the same.

  • Draw FBD
  • Write and Fnet equation and solve.



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