Key Concepts in Physics: Motion & Forces

Comprehensive practice flashcards defining core vocabulary, concepts, formulas, and laws of motion and forces from GCSE Physics.

Magnitude

The size or amount of a physical quantity, such as the size of a force or the measurement of a distance.

Scalar Quantity

A physical quantity that is completely described by its magnitude (size) only, having no direction.

Vector Quantity

A physical quantity that is completely described by both a magnitude (size) and a direction.

Distance

The magnitude of displacement between two positions; a scalar quantity that measures how far an object has moved without considering direction.

Distance Travelled

The total length of the path travelled between two positions; a scalar quantity that can only increase in value.

Displacement

The distance moved in a straight line, in a given direction, from the starting point to the end point; a vector quantity.

Resultant Vector

A single vector quantity that is equivalent to the combined effect of two or more component vectors acting at the same point.

Tip-to-Tail Diagram

A graphical method used to combine two or more vectors together by connecting the tail of subsequent vectors to the tip of previous ones.

Speed

The rate of change of distance travelled ($\text{speed} = \frac{\text{distance travelled}}{\text{time taken}}$); a scalar quantity typically measured in metres per second ($m/s$).

Average Speed

The speed calculated by dividing the total distance travelled over an entire journey by the total time taken ($\text{average speed} = \frac{\text{total distance}}{\text{time taken}}$).

Instantaneous Speed

The speed of a moving object measured over a very short time interval at one particular moment in a journey.

Velocity

The rate of change of displacement; a vector quantity representing the speed of an object in a specified direction.

Acceleration

The rate of change of velocity per second ($a = \frac{v - u}{t}$); a vector quantity measured in metres per second squared ($m/s^2$).

Deceleration

A negative acceleration that occurs when a moving object is slowing down.

Distance-Time Graph Gradient

The slope of the line on a distance-time graph ($\text{gradient} = \frac{\text{rise}}{\text{run}}$), which represents the speed of the moving object.

Velocity-Time Graph Gradient

The slope of the line on a velocity-time graph ($\text{gradient} = \frac{\text{rise}}{\text{run}}$), which represents the acceleration of the moving object.

Area Under a Velocity-Time Graph

The physical geometric area enclosed beneath a velocity-time line, which represents the total distance travelled (or displacement) by the object.

Force

A push or pull that acts on an object due to an interaction with another object, measured in newtons ($N$).

Newton-Meter

A piece of equipment containing a calibrated spring with a hook, used to measure force directly in newtons ($N$).

Contact Force

A force that acts between objects that are physically touching, such as friction, air resistance, tension, and normal reaction forces.

Non-Contact Force

A force that acts at a distance without physical contact between objects due to the action of a field, such as gravity, electrostatics, and magnetism.

Friction

A contact force that opposes the motion of an object and occurs when two touching surfaces rub against each other.

Air Resistance

A specific type of frictional drag force that acts against the direction of an object moving through the air.

Tension

A contact pulling force produced through a length of material (such as a string, cable, or rope) when a force is applied to stretch it.

Reaction Force

A contact supportive force that pushes touching objects apart when an object is supported by a solid surface.

Gravitational Force

An attractive non-contact force experienced between two masses in a gravitational field.

Electrostatic Force

An attractive or repulsive non-contact force experienced between charged particles or objects.

Magnetic Force

An attractive or repulsive non-contact force exerted between the magnetic poles of magnets or moving charges.

Equilibrium

A state in which the vector sum of all forces acting upon an object is equal to zero, meaning no net change in motion occurs.

Resultant Force

The single net force remaining when all individual vector forces acting on an object have been added and resolved.

Balanced Forces

Forces acting on an object in opposite directions that are equal in size, cancelling each other out completely to yield a resultant force of zero ($\Sigma F = 0$).

Unbalanced Forces

Forces acting on an object that do not cancel out completely, producing a non-zero resultant force that causes acceleration.

Free-Body Diagram

A diagram that models all the external forces acting on an isolated object using labelled vector arrows drawn to scale showing their magnitude and direction.

Newton's First Law

The law stating that an object remains at rest or continues to move with a constant velocity in a straight line unless acted upon by a non-zero resultant force.

Newton's Second Law

The law stating that the acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass ($F = ma$).

Newton's Third Law

The law stating that whenever two objects interact, they exert equal and opposite forces on each other of the same type and along the same line of action.

Centripetal Force

The resultant perpendicular force directed towards the centre of a circular path required to maintain an object in uniform circular motion.

Mass

The quantity of matter contained within a physical body; a scalar quantity measured in kilograms ($kg$).

Weight

The downward gravitational force acting on an object's mass ($W = mg$); a vector quantity measured in newtons ($N$).

Centre of Mass

The single point through which the entire weight of an object is considered to act.

Gravitational Field Strength ($g$)

The force of gravity exerted per unit mass, having a value of approximately $9.81\,N/kg$ (frequently rounded to $10\,N/kg$) at the surface of the Earth.

Inertia

The natural tendency of an object to resist any change in its velocity, maintaining its state of rest or uniform motion in a straight line.

Inertial Mass

A measure of how difficult it is to change the velocity of an object, defined mathematically as the ratio of applied force to acceleration ($m = \frac{F}{a}$).

Terminal Velocity

The constant maximum velocity reached by a falling object when the upward resistive force of air resistance balances its downward weight force.

Thinking Distance

The distance travelled by a vehicle during the driver's reaction time, from the moment a hazard is perceived until the brakes are applied.

Braking Distance

The distance travelled by a vehicle from the moment the brakes are pressed until the vehicle comes to a complete halt.

Stopping Distance

The total distance required for a vehicle to come to a complete stop, calculated as the sum of thinking distance and braking distance.

Reaction Time

The time interval taken by an individual to detect a sensory stimulus and initiate a physical response.

Momentum

A vector quantity defined as the product of an object's mass and its velocity ($p = mv$), measured in kilogram metres per second ($kg\,m/s$).

Conservation of Momentum

The principle stating that the total momentum of a closed system before an event or collision is equal to the total momentum after the event, provided no external forces act.

Closed System

A physical system in which no external forces act and no matter or energy is transferred into or out of the system.

Crumple Zone

A structural vehicle safety feature engineered to crush predictably during a collision, increasing impact duration and reducing the resultant force felt by occupants.