# GCSE AQA Physics Unit 3

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• Created on: 29-05-12 22:55

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FORCES AND WAVES
Turning forces and centre of mass:
Moment = Force (n) x perpendicular distance (m)
Centre of mass is directly below point of suspension.
Find CofM by suspend, draw line, suspend, cross line
Balanced moments and stability:
Total anti-clockwise = total clockwise. If equal, the object won't turn.
If the two moments do not equal the same amount, there will be a RESULTANT MOMENT. (it will
turn)
Low and wide objects are most stable. If centre of mass is beyond edge of its base, it will tip over
Circular Motion:
Velocity = speed and direction.
If an object is travelling in a circle, it is accelerating as it is constantly changing its direction.
Force acts towards centre of the circle ­ `centripetal'.
Centripetal force depends on mass speed and radius. If it's faster, force is larger. If it's heavier, force
is larger. If radius is smaller, force is larger as it has more turning.
Gravity and Planetary orbits:
Gravity is the centripetal force in space as it keeps objects in orbit. This is when there is a balance
between forwards motion and inwards motion. It is the force of attraction between masses.
To counteract the stronger gravity, forces nearer the sun move faster, covering orbit quicker.
Size of force of gravity decreases quickly with increasing distance.
Near the sun = smaller orbit, travels faster and in less time.
Artificial satellites for: weather, communications, spying and space research.
Geostationary satellites orbit 24 hours. Stay above same point of earth's surface as it rotates with
them.
Low polar orbit used for weather and spying ­ sweeps over both poles. Few hours per orbit.
Images:
Real image ­ light from object forms image on the screen. Virtual ­ light rays diverge so appears to
be coming from different place.
Describe image: how big compared to object ­ upright or inverted ­ real or virtual.
Diffuse (uneven surface) and clear reflection.
Normal = right angle to the surface! Angle of incidence = angle of reflection <- law of reflection.
Refraction : light waves change direction as they enter a different medium as they change speed.
Mirrors:
How to draw a ray diagram in a plane mirror (image is same size as object, same distance from
mirror, virtual image) :
Draw virtual image (same distance, same size).
Draw reflected ray BOLD FOR EYE TO MIRROR dotted from mirror to image.
Draw incident ray.
Do same for bottom of eye.
Concave ­ shiny on inside of curve. Converges. With convex, it diverges.
Centre of mirror = vertex. Between centre of curvature and vertex = focal point. In front on concave.

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Ray diagram for CONCAVE MIRROR: incident ray parallel to axis pass through focal point when
reflected.
Top of object ­ line parallel to axis to mirror and line passing through focal point to mirror.
I-ray through focal point reflected parallel and I-ray parallel passes through focal point.
Mark 2 reflections. Repeat for bottom.
Distance from mirror effects image ­ At centre of curvature ­ same size. Between c and f ­
bigger. After c ­ smaller. In front of f ­ bigger and tight way.…read more

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High frequency means shorter wavelength.
Ultrasound (frequencies above 20 kHz):
Partially reflected at a boundary between media. Some is reflected back and some is refracted.
Time taken for reflections to show represents distance.
If given speed of sound in medium, can work out distance between boundaries (d = v x t)
Work out speed of ultra sound = frequency x wavelength.
Used to clean some delicate machines as it can be precise and effective. High frequency make
components and dirt vibrate.…read more

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Produces AC voltage.
DYNAMOS ­ they rotate magnet, not coil.
Transformers:
Change AC voltages.
Step-up : more turns in secondary coil than primary. Step down: more turns in primary.
Primary coil produces magnetic field within iron core. Hardly any lost to second coil. Due to AC
current, there is a changing magnetic fields, felt by 2nd coil, inducing alternating voltage.
Number of turns in coils determines if voltage is greater or less.
Iron core transfers the magnetic field, no electricity flows.…read more