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GCSE Unit P3
More Mechanics; Lenses & Mirrors;
Ultrasound; Electromagnetism; Stars.

Summary Notes

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Moments and Centre of Mass

· The weight of a pivoted object can have a turning effect. If the pivot passes through its centre of
mass, the object does not turn, clockwise or anticlockwise. If a force is applied at a distance
from the pivot it has a turning effect…

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Circular Motion

· When an object moves in a circle at a steady speed, the direction of its motion, and thus its
velocity, is constantly changing. Therefore, from Newton's Second Law of Motion, a force
towards the centre of the circle (a centripetal force) is needed. In the case of…

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Reflection
· When a ray of light is reflected from a flat (plane) mirror, or a piece of glass the angle at which
it leaves the surface (angle of reflection) is the same as the angle at which it meets the surface
(angle of incidence).

· Angles are always measured…

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Curved Mirrors

· Curved mirrors work by reflecting light rays according to the rule on page 4; the normal
lines are still at right angles to the surface, but unlike for a plane mirror, are not parallel to
each other

· Curved mirrors are either convex (the back of a…

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Constructing ray diagrams for curved mirrors

1. A clear well-focussed image is formed at a point where two or more rays from the same
point on the object meet up
2. A light ray travelling parallel to the principal axis is reflected by the curved mirror so as to
actually…

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Refraction

· Waves are refracted because they travel at different speeds in different substances (media).

· Rays of light change direction (are refracted) when they cross the boundary between one
transparent substance and another, unless they meet the boundary at right angles (along a
normal).

· Sounds, like light, are…

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Total Internal Reflection

· When a ray of light travels from glass (or any transparent material ­ Perspex, water,
diamond, etc) into air, some of the light is also reflected from the boundary back into the
material.

· If the angle between the ray and a normal is greater than…

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Lenses

· Lenses work by refracting the light at two boundaries according to the rules on page 7. There
are converging and diverging lenses, which cause parallel rays of light incident upon them to
move together/apart after they pass through.

· You should be able to construct ray diagrams to…

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The Eye

· Light emitted or reflected by an object enters the eye through the transparent cornea, where it
undergoes refraction. Together with the lens, the cornea allows a real inverted image to be
produced on the retina. The retina contains millions of rods and cones, which are sensitive to…

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