AQA Physics Unit 3 Revision Notes

Triple science revision notes.

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  • Created on: 25-05-11 18:35
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Turning Forces
A moment is the turning effect of a force:
Moment (Nm) = force (N) x perpendicular distance from the pivot (m).
If the force or perpendicular distance is increased then the moment will be increased.
The centre of mass of an object is the point where its mass may be thought to be
An object will be in equilibrium if it has been suspended then come to rest with its centre of
mass directly below the point of suspension.
The centre of mass of an object is along the axis of symmetry.
The principle of moments states that:
The sum of all the clockwise moments about any point = the sum of all the anticlockwise moments
about that point.
The stability of an object is increased by making its base as wide as possible and its centre of
mass as low as possible.
An object will tend to topple over if the line of action of its weight is outside its base.
For an object moving in a circle at constant speed, the object accelerates continuously
towards the centre of the circle and the centripetal force needed increases, (i) as the mass
or the speed of the object increase (ii) as the radius of the circle decreases.
The force of gravity between two objects is an attractive force, is bigger the greater the
mass of each object is and is smaller the greater the distance between the two objects.
An object is heavier on Earth than on the moon because the Earth's mass is much greater so it
exerts a greater force on an object on its surface.
To stay in orbit at a particular distance, a planet must move at a particular speed around the
The further a planet is from the sun, the less its speed is as it moves round the sun.
Time taken to orbit = circumference/speed
To stay in orbit at a particular distance, a small body must move at a particular speed around
a larger body.
The larger an orbit is, the longer the orbiting body takes to go round the orbit.
The period of a satellite is the time it takes to make one complete orbit.

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A satellite in a geostationary orbit has a period of 24 hours and stays at the same position
directly above the Earths equator.
Geostationary orbits are usually used for communications satellites.
Low polar orbits are used for monitoring satellites.
Light and Sound
The normal at a point on a mirror is perpendicular to the mirror.…read more

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The pitch of a note depends on the frequency of the sound waves.
Ultrasonic waves have a frequency above 18000 kHz. They are partially reflected at a
boundary between two substances and they are non-ionising.
Uses of ultrasonic waves include cleaning devices, flaw detectors and medical scanners.…read more

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High mass star: protostar > main stage> red giant > white dwarf > supernova > neutron star >
black hole if sufficient mass.
Light elements (e.g. iron) are formed inside stars as a result of nuclear fusion.
Heavy elements (e.g. lead) are formed when a massive star collapses than explodes as a
supernova.…read more


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