Editing Reaction (physics) (section)

==Causal misinterpretation==
The terms 'action' and 'reaction' have the misleading suggestion of [[causality]], as if the 'action' is the cause and 'reaction' is the effect. It is therefore easy to think of the second force as being there because of the first, and even happening some time after the first. This is incorrect; the forces are perfectly simultaneous, and are there for the same reason.<ref name=Brown353>{{cite journal
| title = Students' concept of force: the importance of understanding Newton's third law
| last = Brown
| first = David
| journal = Phys. Educ.
| volume = 24
| year = 1989
| issue = 6
| pages = 353–358
| doi = 10.1088/0031-9120/24/6/007
| bibcode = 1989PhyEd..24..353B
| s2cid = 250771986
| quote = Even though one body might be more ‘active’ than the other body and thus might seem to initiate the interaction (e.g. a bowling ball striking a pin), the force body A exerts on body B is always simultaneous with the force B exerts on A.}}</ref>

When the forces are caused by a person's volition (e.g. a soccer player kicks a ball), this volitional cause often leads to an asymmetric interpretation, where the force by the player on the ball is considered the 'action' and the force by the ball on the player, the 'reaction'. But physically, the situation is symmetric. The forces on ball and player are both explained by their nearness, which results in a pair of contact forces (ultimately due to electric repulsion). That this nearness is caused by a decision of the player has no bearing on the physical analysis. As far as the physics is concerned, the labels 'action' and 'reaction' can be flipped.<ref name=Brown353/>

==='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.

===Centripetal and centrifugal force===
Another common mistake is to state that "the centrifugal force that an object experiences is the reaction to the centripetal force on that object."<ref>{{Citation
 | last =Adair
 | first =Aaron
 | title =Student Misconceptions about Newtonian Mechanics: Origins and Solutions through Changes to Instruction
 | year =2013
 | publisher =The Ohio State University
 | bibcode =2013PhDT.......476A
 | url =http://rave.ohiolink.edu/etdc/view?acc_num=osu1386034522
 | quote = This was attacked by Newton who tried to have the centripetal force on the planets (from gravitational interactions) be matched by the centrifugal force so there would be a balance of forces based on his third law of motion}}</ref><ref name=Aiton268a>{{Citation
 | first =Eric
 | last =Aiton
 | editor-last =Swetz
 | editor-first =Frank
 | display-editors =et al.
 | title =An Episode in the History of Celestial Mechanics and its Utility in the Teaching of Applied Mathematics
 | series =Learn from the Masters
 | year =1995
 | publisher =The Mathematical Association of America
 | isbn =978-0883857038
 | quote =... in one of his attacks on Leibniz written in 1711, Newton says that centrifugal force is always equal and opposite to the force of gravity by the third law of motion.
 | url-access =registration
 | url =https://archive.org/details/learnfrommasters0000unse
 }}</ref>

If an object were simultaneously subject to both a [[centripetal force]] and an equal and opposite [[centrifugal force]], the [[Net force|resultant force]] would vanish and the object could not experience a circular motion. The centrifugal force is sometimes called a [[fictitious force]] or pseudo force, to underscore the fact that such a force only appears when calculations or measurements are conducted in non-inertial reference frames.<ref>{{Citation
 | last =Singh
 | first =Chandralekha 
 | title =Centripetal Acceleration: Often Forgotten or Misinterpreted
 | journal =Physics Education
 | volume =44
 | issue =5
 | pages =464–468
 | year =2009
 | doi =10.1088/0031-9120/44/5/001
 | quote =Another difficulty is that students often consider the pseudo forces, e.g., the centrifugal force, as though they were real forces acting in an inertial reference frame. | arxiv =1602.06361
 | bibcode =2009PhyEd..44..464S 
 | s2cid =118701050 
 }}</ref>