Rotational motion and astrophysics

SQA AH Physics Unit 1 - Key Terms

Tangential

A line touching at one point, acting at 90 degrees to a particular point

Centripetal

Moving towards or in a central direction

Period (T)

The time for one revolution measured in seconds

Frequency (f)

The number of revolutions per second measured in hertz

Angular acceleration

The time rate of change or angular velocity of a rotating body, denoted by the Greek letter alpha

Interia

A property of matter which resists change in motion or movement

Moment of inertia (SQA term)

A measure of an object’s resistance to angular acceleration about a given axis

Torque

A force acting in a perpendicular direction that causes an object to rotate about an axis

Angular momentum

The quantity of rotation of a body. It is given as the product of the moment of intertia and angular velocity. If no external torque acts on a system, the angular momentum of the system will remain constant. This is known as the principle of conservation of angular momentum.

Astronomical unit (AU)

Roughly the mean distance between the Earth and the Sun

Gravitational field

A region where an object experiences a force due to its mass

Gravitational field strength (SQA term)

The gravitational force acting on a unit mass

Satellite

An object which orbits a larger mass

Gravitational potential (SQA term)

The energy an object of mass has at a point in relation to another massive object due to gravity

Escape velocity (SQA term)

The minimum velocity required to allow a mass to escape a gravitational field to infinity, where the mass achieves zero kinetic energy and maximum (zero) potential energy

Frames of reference

This refers to a co-ordinate and reference system by which we can determine position and velocities

Intertial frames of reference

One where the observer is stationary or moving at a constant speed.

Non-intertial frames of reference

One where the observer is accelerating

Equivalence principle

It is indistinguishable for an observer to tell if they are in an accelerating object or in a gravitational field

Spacetime

A model where the three dimensions of space are combined with the dimension of time

Black hole

An incredibly massive object which curves spacetime so that the escape velocity is greater than the speed of light

Event horizon

The boundary of a black hole at which the escape velocity is equal to the speed of light. An event horizon can be defined as the point from which nothing can return. From the perspective of a distant observer, time appears to be frozen at the event horizon of a black hole.

Schwarzchild radius

The distance from its centre (singularity) to its event horizon

Protostar

The embryonic early phase of a star

Luminosity

The total amount of electromagnetic energy (of all forms) radiated by a star

Apparent brightness

A slightly more subjective description of the luminosity of a star as it refers to the amount of energy reaching the observer which depends also on the distance of the star from us

Proton-proton chain reaction

The nuclear fusion reaction which occurs in our Sun and other stars with core temperatures less than 15 million kelvin