# Physics unit 3 notes, AQA

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P3 Revision ­ Turning Forces
Give the equation for moments
Moments = Force x Perpendicular Distance from the pivot to the line of action
Describe the centre of mass
The centre of mass is the point where an object's mass is concentrated
Define the Principle of Moments
The Principle of Moments states that to be in equilibrium, the sum of the anticlockwise
moments about a point is equal to the sum of the clockwise moments about that point
Give factors that would increase the stability of an object
If the line of action is outside the base, it will topple
o A wide base
o A low centre of mass
Define the centripetal force
The centripetal force always acts towards the centre of a circle, causing the object to
accelerate
Give factors that would increase the centripetal force
A circle with a smaller radius
An object with a higher speed
An object with a larger mass
Define the force of gravity
Gravity causes each body in the universe to attract every other body
Give factors that would increase the force of gravity
An object with a larger mass
An smaller distance between the objects
Describe what must happen for a small object to stay in orbit
To stay in orbit at a particular distance, a small body must move at a particular speed around
a larger body
Describe the effect on a planet's orbit as a result of being further away from the
sun

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The planet will travel more slowly around the sun
The planet will take longer to make a compete orbit
Explain the difference between geostationary and polar orbits
Geostationary orbits:
Remain in the same place above the Earth's surface using an orbit of 24 hours above
the equator and are used for communication satellites.…read more

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Cameras use a converging lens to produce a real image whereas magnifying glasses use a
converging lens to produce a virtual image
Describe a sound wave
A sound wave is caused by mechanical vibrations and travels as a wave
Explain the differences and similarities between light and sound
Differences:
Light waves are transverse (the vibrations of the wave travel at right angles to the
wave) whereas sound waves are longitudinal (the vibrations of the wave travel in the
same direction to the wave)

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Reverse the direction of the magnetic field
Explain the importance of the motor effect in a motor
When a current passes through the coil, a force acts on each side of the coil so the
coil spins due to the motor effect.
This force on each side of the coil is in the opposite direction to the force on other

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When an alternating current passes through the primary coil, an alternating magnetic
field is produced which continually expands and contracts.
The alternating magnetic field lines pass through the secondary coil and induce an
alternating potential difference across its ends as well as a current if there is a
compete current.
Explain why transformers only work using alternating current
If a dc current is used, a magnetic field is produced but it would not continually
contract and expand so no potential difference would be induced.…read more

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Describe the life cycle of a star
Clouds of dust and gas gather by gravitational attraction, merging to become
increasingly dense and form a protostar.
As a protostar becomes denser, it gets hotter until the nuclei of hydrogen atoms
and other light elements fuse together. Energy is released during the process and
the core gets hotter, brighter and shines.…read more

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Radio telescopes have been used to look for signals from space to indicate existence