passage: What if two children push a swing at the same time ?
  Would the swing go higher ?
  Yes , together their force would be more .
  It is common for forces to be combined .
  Most objects on Earth have at least two forces acting on them .
  Do you know what one of them is ?
  Of course , that force is gravity .
  How many forces do you have on you right now ?
  Gravity pulls you down toward the center of Earth .
  Your legs exert a downward force .
  They hold you up against the pull of gravity .
  Consider the example in Figure 1.3 .
  A book is resting on a table .
  Gravity pulls the book downward .
  It has a force of 20 newtons .
  At the same time , the table pushes the book upward .
  Its force is also 20 newtons .
  The table opposes the pull of gravity .
query_and_answer: Is it rare for objects on Earth to have at least two forces acting on them ? || No
evidences: It is common for forces to be combined . Most objects on Earth have at least two forces acting on them .