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31 terms

Key words and definitions for Unit G481 (Mechanics) of OCR Physics A (AS Level). Created by Rob Cowen, Head of Physics (www.cowenphysics.com).

newton

The force which gives a mass of 1kg an acceleration of 1ms^-2.

joule

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

watt

The power transferred by one joule per second.

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

mass / volume

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

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

= force / area

strain

= extension / original length

Young modulus

= stress / strain, 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