Editing Reaction (physics) (section)

==='Equal and opposite'===
One problem frequently observed by physics educators is that students tend to apply Newton's third law to pairs of 'equal and opposite' forces acting on the same object.<ref>{{cite journal
| title = Alternative frameworks: Newton's third law and conceptual change
| author = Colin Terry and George Jones
| journal = European Journal of Science Education
| volume = 8
| issue = 3
| year = 1986
| pages =291–298
| quote = This report highlights some of the difficulties that children experience with Newton's third law.
| doi=10.1080/0140528860080305|bibcode = 1986IJSEd...8..291T }}</ref><ref>{{cite journal
| title = Newton's Third Law Revisited
| author = Cornelis Hellingman
| journal = Physics Education
| volume = 27
| issue = 2
| pages = 112–115
| year = 1992
| quote = ... following question in writing: Newton’s third law speaks about ‘action’ and ‘reaction’. Imagine a bottle of wine standing on a table. If the gravitational force that attracts the bottle is called the action, what force is the reaction to this force according to Newton’s third law? The answer most frequently given was: ‘The normal force the table exerts on the bottle’.
| doi=10.1088/0031-9120/27/2/011|bibcode = 1992PhyEd..27..112H | s2cid = 250891975
}}</ref><ref name=French314>{{Citation  | last = French  | first = Anthony  | title = Newtonian Mechanics  | year = 1971|pages=314|quote=… Newton’s third law, that ‘‘action and reaction are equal and opposite’’}}</ref>
This is incorrect; the third law refers to forces on two different objects. In contrast, a book lying on a table is subject to a downward gravitational force (exerted by the earth) and to an upward normal force by the table, both forces acting on the same book. Since the book is not accelerating, these forces must be exactly balanced, according to Newton's second law. They are therefore 'equal and opposite', yet they are acting on the same object, hence they are not action-reaction forces in the sense of Newton's third law. The actual action-reaction forces in the sense of Newton's third law are the weight of the book (the attraction of the Earth on the book) and the book's upward gravitational force on the earth.  The book also pushes down on the table and the table pushes upwards on the book.
Moreover, the forces acting on the book are not always equally strong; they will be different if the book is pushed down by a third force, or if the table is slanted, or if the table-and-book system is in an accelerating elevator. The case of any number of forces acting on the same object is covered by considering the sum of all forces.

A possible cause of this problem is that the third law is often stated in an abbreviated form: ''For every action there is an equal and opposite reaction,''<ref>{{cite web
 | url =https://www.grc.nasa.gov/www/k-12/airplane/newton3.html
 | title =Newton's Third Law Applied to Aerodynamics
 | last =Hall
 | first =Nancy
 | publisher =NASA
 | archive-url =https://web.archive.org/web/20181003133844/https://www.grc.nasa.gov/www/k-12/airplane/newton3.html
 | archive-date =2018-10-03
 | quote =for every action (force) in nature there is an equal and opposite reaction }}</ref> without the details, namely that these forces act on two different objects. Moreover, there is a causal connection between the weight of something and the normal force: if an object had no weight, it would not experience support force from the table, and the weight dictates how strong the support force will be. This causal relationship is not due to the third law but to other physical relations in the system.