# OCR Physics A Mechanics (G481) definitions

### Definitions for OCR Physics A – G481 Mechanics

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#### Prefixes

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