Definitions for Module 3 – OCR A Physics


femto – \times 10^{-15}
pico – \times 10^{-12}
nano – \times 10^{-9}
micro – \times 10^{-6}
milli – \times 10^{-3}
centi – \times 10^{-2}
unit – \times 10^{0} ( \times 1)
kilo – \times 10^{3}
mega – \times 10^{6}
giga – \times 10^{9}
tera – \times 10^{12}



The unit of force which gives a mass of 1 \ kg  an acceleration of 1 \ ms^{-2}


The unit of work done by a force of 1\ N  moving an object 1 \ m  in the direction of the force


The unit of power transferred by 1 \ J  per second.


An alternative unit for measuring angles
(2 \pi \text{[rad]} = 360^{ \circ }  )



The distance moved by an object in a particular direction

Instantaneous speed

The speed at the precise instant that it is measured (e.g a cars speedometer)

Average Speed

The speed calculated during a period of time


The rate of change of distance;
\text{speed} = \frac{\text{Change of distance}}{\text{Time}}


The rate of change of displacement;
\text{velocity} = \frac{\text{Change of displacement}}{\text{Time}}


The rate of change of velocity;
\text{Acceleration} = \frac{\text{Change of velocity}}{\text{Time}}


The ratio of mass to volume;
\text{Density} = \frac{\text{Mass}}{\text{Volume}}


Force per unit area

Torque of a couple

One of the force multiplied by the perpendicular distance between the forces.

Moment of a force

Force time the perpendicular distance from the pivot

Work done by a force

Force times the distance moved in the direction of the force


The rate of doing work;
\text{Power} = \frac{\text{Work done}}{\text{time}}

Linear momentum

The product of mass and velocity

Net force on a body

Rate of change of its momentum

Impulse of a force

Force multiplied by the time for which the force acts

Force constant of a spring

Force per unit extension



A quantity with magntiude only


A quantity with both magnitude and direction

Terminal Velocity

The velocity of a falling object when \text{weight} = \text{drag}

Centre of Gravity

A point where the entire weigh of an object appears to act

Thinking distance

The distance traveled by a vehicle in the time taken by the driver to react

Braking Distance

The distance traveled by a vehicle after the brakes are applied and until the vehicle stops

Stopping distance

\text{Stopping distance} = \text{Thinking distance} + \text{Braking distance}

Principle of conservation of energy

Energy cannot be created of destroyed; it can only be transferred into other forms

Principle of moments

For an object in equilibrium, the sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments about the same point

Newton’s first law

A body will remain at rest or continue to move with constant velocity unless acted upon by a force

Newton’s second law

The rate of change of momentum is equal to the net force acting and takes place in the direction of that force

Newton’s third law

When one body exerts a force on another, the other body exerts an equal (in magnitude) and opposite (in direction) force on the first body


Resultant force is zero

Principle of conservation of momentum

Total momentum is constant for a closed system

Perfectly elastic collision

A collision with no loss of kinetic energy

Inelastic collision

A collision with some loss of kinetic energy

Hooke’s Law

Extension of a spring is proportional to the force applied, as long as the elastic limit is not exceeded


\text{Stress} = \frac{\text{Force}}{\text{Area}}


\text{Strain} = \frac{\text{Extension}}{\text{Original length}}

Young Modulus

\text{Young Modulus} = \frac{\text{Stress}}{\text{Strain}}

Ultimate tensile stress

The breaking stress of a material

Elastic deformation

Material returns to its original shape when the force is removed

Plastic deformation

Material does not return to its original shape when the force is removed