### Definitions for OCR Physics A – G481 Mechanics

**Learn and practice your knowledge of these definitions using Quizlet.**

#### 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