Wednesday, 21 October 2020, 1:15 AM
Site: The Science Portal: 21C Science with Style
Course: The Science Portal: 21C Science with Style (21CScience)
Glossary: IB Physics Glossary
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#### Natural frequency

is the frequency at which an object will vibrate if disturbed. (d)

#### Near point

is the closest point to the eye on which the eye can focus.

#### Newton's first law of motion

an object remains stationary or remains at a constant velocity if there is no resultant force acting on the object. (s)

#### Newton's Law of Gravitation

$$F = G \frac{m_1 m_2}{r^2}$$
where F is the force exerted by object 1 on object 2 (and vice versa), G is the gravitational constant (6.67 x 10-11 Nm2kg-2), m1 is the mass of object 1, m2 is the mass of object 2 and r is the distance between the centres of the objects.

We assume the masses are point-like and all the masses is concentrated in the center. An approximation is to assume the separation of the masses, r, is much greater than their radii.

The law is an inverse-square-law, $$F \propto \frac{1}{r^2}$$.

#### Newton's second law of motion

states, a resultant force acting on a body equals the rate of change of momentum of the body, $$F=\frac{\Delta p}{\Delta t}=\frac{m\Delta v}{\Delta t} = m \times a$$. (s)

#### Newton's third law of motion

when two bodies A and B interact, the force that A exerts on B is equal and opposite to the force that B exerts on A. (s)

#### Node

a region of zero wave displacement.

#### Nuclear fission

is a nuclear reaction in which large nuclei are induced to break up into small nuclei and release energy in the process.
An example of nuclear fission
$$^{1}_{0}n + ^{235}_{92}U -> ^{236}_{92}U -> ^{144}_{56}Ba + ^{89}_{36}Kr + 3 ^{1}_{0}n$$.

#### Nuclear fusion

is a nuclear reaction in which small nuclei are induced to join together into larger nuclei and energy is released in the process. Nuclear fusion is the main source of the Sun’s energy. An example of hydrogen fusion:
$$^{2}_{1}H + ^{2}_{1}H -> ^{3}_{2}He + ^{1}_{0}n$$.

#### Nuclear model

A simple nuclear model of the atom consists of a tiny central nucleus containing all the mass and all the positive charge. The nucleus is made up of protons and neutrons. Negative electrons are kept in orbit around the nucleus as a result of the electrostatic attraction between the electrons and the nucleus. Although this simple model helps explain many atomic properties there are reasons why things cannot be this simple.