Newton’s laws, or more accurately, equations or rules of motion and gravitation are accepted and widely used and essentially engineering problems. Except that while we have gravity described by a formula, we actually don’t understand how gravity operates. There is also the problem of applying Newton’s Laws where they should not be applied such as geophysics.
Newton’s laws are limited to independent objects separated by ‘space’; apple falling off tree to Earth or Moon orbiting Earth or airplane flying over the Earth. If there is no space between the two objects then we can not observe any motion between them. Hence these objects are not Newtonian objects.
Therefore Newton’s equations do not apply between the constituent parts of the same object since those parts are not independent objects capable of independent motion.
Perhaps we should dethrone Newton and call is ideas as his Rules of Motion?
Update: Newton had to deal with a competing explanation:
PUSHING GRAVITY: New Perspectives on Le Sage’s Theory of Gravitation
Editor: Matthew R. EDWARDS. Apeiron, Montreal. English, 316p, U$25.00. Can be ordered at
redshift.vif.com or http://www.amazon.com.