DBE Grade 12 Physical Sciences Paper 1 — Definitions, Laws and Theorems

Verbatim definitions, laws, and principles for Grade 12 Physical Sciences Paper 1 (Physics) extracted directly from the 2021 DBE Exam Guidelines.

Normal force

The force or the component of a force which a surface exerts on an object with which it is in contact, and which is perpendicular to the surface.

Frictional force

The force that opposes the motion of an object and which acts parallel to the surface.

Static frictional force

The force that opposes the tendency of motion of a stationary object relative to a surface.

Kinetic frictional force

The force that opposes the motion of a moving object relative to a surface.

Free-body diagram

A diagram that shows the relative magnitudes and directions of forces acting on a body/particle that has been isolated from its surroundings.

Newton's first law of motion

A body will remain in its state of rest or motion at constant velocity unless a non-zero resultant/net force acts on it.

Newton's second law of motion

When a net force acts on an object, the object will accelerate in the direction of the force and the acceleration is directly proportional to the force and inversely proportional to the mass of the object.

Newton's third law of motion

When object A exerts a force on object B, object B SIMULTANEOUSLY exerts an oppositely directed force of equal magnitude on object A.

Newton's Law of Universal Gravitation

Each body in the universe attracts every other body with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

Weight

The gravitational force the Earth exerts on any object on or near its surface.

Momentum

The product of an object's mass and its velocity.

Linear momentum

A vector quantity with the same direction as the velocity of the object.

Newton's second law of motion in terms of momentum

The net (or resultant) force acting on an object is equal to the rate of change of momentum of the object in the direction of the net force.

Impulse

The product of the resultant/net force acting on an object and the time the net force acts on the object.

Isolated system

A system on which the net external force is zero.

Principle of conservation of linear momentum

The total linear momentum of an isolated system remains constant (is conserved).

Projectile

An object which has been given an initial velocity and then it moves under the influence of the gravitational force only.

Free fall

Motion during which the only force acting on an object is the gravitational force.

Work done on an object by a constant force

Defined as $F \Delta x \cos(\theta)$, where $F$ is the magnitude of the force, $\Delta x$ the magnitude of the displacement and $\theta$ the angle between the force and the displacement.

Work-energy theorem

The work done on an object by a net force is equal to the change in the object's kinetic energy OR the work done on an object by a net force is equal to the change in the object's kinetic energy (in symbols: $W_{\text{net}} = \Delta K = K_f - K_i$).

Conservative force

A force for which the work done in moving an object between two points is independent of the path taken.

Non-conservative force

A force for which the work done in moving an object between two points depends on the path taken.

Principle of conservation of mechanical energy

The total mechanical energy (sum of gravitational potential energy and kinetic energy) in an isolated system remains constant.

Power

The rate at which work is done or energy is expended (in symbols: $P = \frac{W}{\Delta t}$).

Doppler effect (sound)

The change in frequency (or pitch) of the sound detected by a listener, because the sound source and the listener have different velocities relative to the medium of sound propagation.

Coulomb's law

The magnitude of the electrostatic force exerted by one point charge ($Q_1$) on another point charge ($Q_2$) is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance ($r$) between them.

Electric field

A region of space in which an electric charge experiences a force.

Electric field at a point

The electrostatic force experienced per unit positive charge placed at that point (in symbols: $E = \frac{F}{q}$).

Ohm's law

The potential difference across a conductor is directly proportional to the current in the conductor at constant temperature.

Kilowatt hour

Refers to the use of $1\,\text{kilowatt}$ of electricity for $1\,\text{hour}$.

emf

The maximum energy provided by a battery per unit charge passing through it.

rms potential difference

The AC potential difference which dissipates/produces the same amount of energy as an equivalent DC potential difference.

rms current

The alternating current which dissipates/produces the same amount of energy as an equivalent direct current (DC).

Photoelectric effect

The process whereby electrons are ejected from a metal surface when light of suitable frequency is incident on that surface.

Threshold frequency ($f_0$)

The minimum frequency of light needed to emit electrons from a certain metal surface.

Work function ($W_0$)

The minimum energy that an electron in the metal needs to be emitted from the metal surface.

Atomic absorption spectrum

A spectrum formed when certain frequencies of electromagnetic radiation passing through a substance is absorbed.

Atomic emission spectrum

A spectrum formed when certain frequencies of electromagnetic radiation are emitted due to an atom making a transition from a higher energy state to a lower energy state.