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Another candidate for a "zeroth law" is the fact that at any instant, a body reacts to the forces applied to it at that instant. [36] Likewise, the idea that forces add like vectors (or in other words obey the superposition principle ), and the idea that forces change the energy of a body, have both been described as a "fourth law".
The normal force, for example, is responsible for the structural integrity of tables and floors as well as being the force that responds whenever an external force pushes on a solid object. An example of the normal force in action is the impact force on an object crashing into an immobile surface.
This force is applied in a direction opposite to gravitational force, that is of magnitude: B = ρ f V disp g , {\displaystyle B=\rho _{f}V_{\text{disp}}\,g,\,} where ρ f is the density of the fluid, V disp is the volume of the displaced body of liquid, and g is the gravitational acceleration at the location in question.
In physics, Hooke's law is an empirical law which states that the force (F) needed to extend or compress a spring by some distance (x) scales linearly with respect to that distance—that is, F s = kx, where k is a constant factor characteristic of the spring (i.e., its stiffness), and x is small compared to the total possible deformation of the spring.
This support force is an 'equal and opposite' force; we know this not because of Newton's third law, but because the object remains at rest, so that the forces must be balanced. To this support force there is also a 'reaction': the object pulls down on the supporting cable, or pushes down on the supporting surface or liquid.
Newton's law of motion for a particle of mass m written in vector form is: = , where F is the vector sum of the physical forces applied to the particle and a is the absolute acceleration (that is, acceleration in an inertial frame) of the particle, given by: = , where r is the position vector of the particle (not to be confused with radius, as ...
It is useful to notice that the resultant force used in Newton's laws can be separated into forces that are applied to the particle and forces imposed by constraints on the movement of the particle. Remarkably, the work of a constraint force is zero, therefore only the work of the applied forces need be considered in the work–energy principle.
The argument is as follows. The principle of virtual work states that in equilibrium the virtual work of the forces applied to a system is zero. Newton's laws state that at equilibrium the applied forces are equal and opposite to the reaction, or constraint forces. This means the virtual work of the constraint forces must be zero as well.