N

Neutrino
An almost massless particle of neutral charge that is released along with a beta particle in beta decay.
Neutron
A neutrally charged particle that, along with protons, constitutes the nucleus of an atom.
Neutron number
The number, N, of neutrons in an atomic nucleus.
Newton
A unit of force: 1 N is equivalent to a 1 kg · m/s2.
Newton’s First Law
An object at rest remains at rest, unless acted upon by a net force. An object in motion remains in motion, unless acted upon by a net force.
Newton’s Law of Universal Gravitation
The force of gravity, F, between two particles of mass and , separated by a distance r, has a magnitude of , where G is the gravitational constant. The force is directed along the line joining the two particles.
Newton’s Second Law
F = ma. The net force, F, acting on an object causes the object to accelerate, a. The magnitude of the acceleration is directly proportional to the net force on the object and inversely　proportional to the mass, m, of the object.
Newton’s Third Law
To every action, there is an equal and opposite reaction. If an object A exerts a force on another object B, B will exert on A a force equal in magnitude and opposite in direction to the force exerted by A.
Node
The points on a standing wave where total destructive interference causes the medium to remain fixed at its equilibrium position.
Normal
The line perpendicular to a surface. There is only one normal for any given surface.
Normal force
The reaction force of the ground, a table, etc., when an object is placed upon it. The normal force is a direct consequence of Newton’s Third Law: when an object is placed on the ground, the ground pushes back with the same force that it is pushed upon. As a result, the net force of an object on the ground is zero, and the object does not move.
Nuclear fission
A nuclear reaction in which a high-energy neutron bombards a heavy, unstable atomic nucleus, causing it to split into two smaller nuclei, and releasing some neutrons and a vast amount of energy at the same time.
Nuclear fusion
A nuclear reaction that takes place only at very high temperatures. Two light atoms, often hydrogen, fuse together to form a larger single atom, releasing a vast amount of energy in the process.
Nucleus
The center of an atom, where the protons and neutrons reside. Electrons then orbit this nucleus.