# Unit 2 Definitions

Moment
Force multiplied by perpendicular distance to pivot.
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Principle of Moments
For an object to be in equilibrium, the sum of the anticlockwise moments must equal the sum of the clockwise moments.
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Couple
A pair of opposite and equal forces separated by distance d, acting about point p, the turning effect at point p is equal to one of the forces multiplied by the perpendicular distance between the forces.
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Centre of Mass
Where all of an object's mass appears to act and moments cancel.
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One Newton
Force required to accelerate 1kg by 1ms^-2.
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One Joule
The work done by a force of one newton when the point where the force is applied moves one metre in the direction of the force.
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Law of Conservation of energy
Energy is not created or destroyed, just converted between different types.
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Hooke's Law
The extension of a spring is proportional to the force applied, as long as the limit of proportionality is not exceeded.
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Stress
Force per unit area.
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Strain
Ratio of extension to original length.
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Young's Modulus
Stiffness constant of a material.
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Longitudinal Wave
The displacement of the medium is in the same direction as the direction of travel of the wave.
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Transverse Wave
The displacement of the medium is perpendicular to the direction of travel of the wave.
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Stationary/Standing Wave
When two waves of similar frequency, travelling in opposite directions, interfere resulting in places with very large continuous amplitude oscillation while others have zero amplitude and continuous destructive interference.
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Diffraction
Spreading of a wave when is passes through a gap or past an obstacle.
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Coherent Sources
Same frequency and constant phase relationship.
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Single Slit Diffraction
Wide central maximum, twice the width of others, reduction in intensity moving away from centre.
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Double Slit Diffraction
Same as single slit, but with smaller fringes within each maximum.
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## Other cards in this set

### Card 2

#### Front

Principle of Moments

#### Back

For an object to be in equilibrium, the sum of the anticlockwise moments must equal the sum of the clockwise moments.

Couple

Centre of Mass

One Newton