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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, [8] without the details, namely that these forces act on two different objects ...
For example, consider a book at rest on a table. The Earth's gravity pulls down upon the book. The "reaction" to that "action" is not the support force from the table holding up the book, but the gravitational pull of the book acting on the Earth. [note 6] Newton's third law relates to a more fundamental principle, the conservation of momentum.
The action is defined by an integral, and the classical equations of motion of a system can be derived by minimizing the value of that integral. The action principle provides deep insights into physics, and is an important concept in modern theoretical physics. Various action principles and related concepts are summarized below.
The action depends on the energy function, and the energy function depends on the position, motion, and interactions in the system: variation of the action allows the derivation of the equations of motion without vector or forces. Several distinct action principles differ in the constraints on their initial and final conditions.
There are two main descriptions of motion: dynamics and kinematics.Dynamics is general, since the momenta, forces and energy of the particles are taken into account. In this instance, sometimes the term dynamics refers to the differential equations that the system satisfies (e.g., Newton's second law or Euler–Lagrange equations), and sometimes to the solutions to those equations.
The external forces: These are indicated by labelled arrows. In a fully solved problem, a force arrow is capable of indicating the direction and the line of action [notes 1] the magnitude; the point of application; a reaction, as opposed to an applied force, if a hash is present through the stem of the arrow
Force is the action of one body on another. A force is either a push or a pull, and it tends to move a body in the direction of its action. The action of a force is characterized by its magnitude, by the direction of its action, and by its point of application (or point of contact). Thus, force is a vector quantity, because its effect depends ...
Action at a distance is the concept in physics that an object's motion can be affected by another object without the two being in physical contact; that is, it is the concept of the non-local interaction of objects that are separated in space.