# OCR Physics A Mechanics (G481) definitions

### Definitions for OCR Physics A – G481 Mechanics

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pico
nano
micro
milli
centi
kilo
mega
giga
tera

#### Units

newton – The force which gives a mass of an acceleration of .
joule – The work done by a force of moving an object in the direction of the force.
watt – The power transferred by one joule per second.

#### Quantities

displacement – 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 car’s speedometer).
average speed – the speed calculated during a period of time.
speed – rate of change of distance ().
velocity – rate of change of displacement ().
acceleration – rate of change of velocity ().
density
torque of a couple – one of the forces x perpendicular distance between the forces.
moment of a force – force x perpendicular distance from pivot.
work done by a force – force x distance moved in the direction of the force.
power – the rate of work done ().
force constant of a spring – force per unit extension

#### Concepts

scalar – a quantity with magnitude only.
vector – a quantity with both magnitude and direction.
terminal velocity – the velocity of a falling object when weight = drag.
centre of gravity – a point where the entire weight of an object appears to act.
thinking distance – the distance travelled by the car in the time taken by the driver to react.
braking distance – the distance travelled by the car after the brakes are applied until the car stops.
stopping distance – thinking distance + braking distance.
principle of conservation of energy – Energy cannot be created or 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 = the sum of the anticlockwise moments about the same point.
Hooke’s law – extension of a spring is proportional to the force applied (as long as the elastic limit is not exceeded).
stress.
strain.
Young modulus as long as the elastic limit is not exceeded.
ultimate tensile strength – the breaking stress of a material.
elastic deformation – material returns to original shape when the force is removed.
plastic deformation – material does not return to original shape when the force is removed.

#### Graphs

Brittle and ductile

Polymeric