# AQA GCSE P3 Physics

Every topic covered in P3. Short, concise notes.

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• Created by: cloe
• Created on: 25-05-11 20:03

## Moments and Centre of Mass

Moments

Moment (Nm) = force (N) x distance from pivot (m)

If the anitclockwise moment = clockwise moment, the object is stable.

To increase the moment, increase the force used, or the distance from the pivot (eg using a spanner to tun a bolt).

Centre of Mass

Centre of Mass: the point at which the total mass in an object appears to act.

To find an objects centre of mass:

• Tie the object from a corner to a piece of string and put a plumline (weighted piece of string) on the other end.
•  Hang this over something (eg a nail in a wall/clamp stand).
• Because there is no moment, the plubline hangs straight through the centre of mass. Draw this line along the object (tricky)
• Repeat but rotate object. Draw another line - where these cross is the centre of mass.

http://www.school-for-champions.com/science/images/gravity_center_plumb_line.gif

This picture shows the objects hanging and the lines drawn on.

(I do not own any pictures)

Stability

Stable: Balanced. centre of mass below the object => no turning force

Unstable: Will not balance as centre of mass is not beneath object => a turning force

Neutral: can easily be knocked over

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## Circular Motion

Centripetal Force

For an object moving in a circle at a constant speed:

- the object accelerates towards the centre of the circle constantly

- the centripetal force increases as:

the mass or the speed increases

the radius of the circle decreases

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## Orbits

Orbits

Mass: the amount of matter in an object

Weight: the force of gravity acting upon an object with a mass. Measured in Newtons

Circular orbits have one focus point

Elliptical orbits have 2 focus points. The sum of the distance between them is equal

Satellites

Geostationary: orbits at a certain height above the equator with the same speed as the earth. Looks stationary.

Low polar orbit: orbits over the poles at a low height. Used for monitoring.

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## Mirrors

Reflection

tall, thin image = convex [bends out  ()  ]

short, broad image = concave [bends in  )(  ]

Mirror image: refelction of light

The normal at a point on a mirror is perpendicular to the mirror

For a light ray reflected by a mirror angle of incidence ray = angle of reflection

Real images are used in films

Virtual images cannot be projected

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## Lenses

Concave Lens

principal focus is at the point the parallel rays are focused by the mirror

forms a real image if the object is beyond the principal focus

or a virtual image if the object is between the mirror and the principal focus

Convex Lens

ALWAYS forms a virtual image ()

Refraction is when an object changes the speed light is travelling at as it goes through (it slows down)

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## Lenses (II)

Converging lens

Parallel light rays converge (meet) at the principal focus. Any image is upside down.

Diverging Lens

Parallel light rays diverge (spread out). Tthe principal focus is where the ray leaves the lens

Focal length: distance from principal focus to the centre of the lens.

Cameras:

contain converging lens to get a real image

contain film/pixels that pick up light and form image

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## Sound

Sound

can travel through liquids, gases and solids      Human ear range: 20-20,000 Hz

can't travel through a vacuum

are longitudinal waves (across => )

can be reflected (echoed) and refracted

• Pitch increases as frequency increase
• Loudness increases as amplitude (height) increases
• Quality of sound depends on the waveform

Shape of sound wave: triangle; weak and mellow. Sqaure; hollow. Sawtooth ( /|/|/| ); buzzy, brassy. Pulse: thin ready sound.

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## Ultrasound

Ultrasound:

20,000 Hz + (out of human range)

Electronic systems can produce them

Partially refelcted when they meet a boundary between 2 different mediums

Time taken for reflections to reach detector is how far away the boundary is

Can be used for industrial cleaning and quality control

Frequency = 1/Time (secs)
Speed = freq (Hz) x wavelength (m)

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## Motor Effect

If a wire cuts across a magnetic field, potential difference/a current is induced.

Direction of flux, current and motion are at right angles to each other.

Increased 'flux cutting' can be done by:
a stronger magnet
varying the magnetic field (faster)
moving the coil of wire (faster)

DYNAMO EFFECT: produces electricity from the motion (induction)

LEFT HAND RULE:

• ThuMb = Motion
• First = Field
• SeCond = Current

[thumb up, first finger out (like a gun) second figner at right angle to first finger. If confusing, Google 'Newton's left hand rule' for a picture]

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## Motor effect contd. and Electricity

When the coil of wire cuts perpendicular through the magnetic field, it induces a current. Because it cuts the magnetic fierld and the wire is attatched to a circuit, the current that was induce can travel through the complete circuit, powering a motor.

Step up-step downTransformers:

Step up: from power station to pylons. More efficient, less lost as heat and over the distance travelled.

Stepdown: from pylons into homes. Appliences can't handle such voltage, average plug voltage in EU 240v.

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## Stars

Stars:

• A star to be is called a protostar. Particles of dust and gas gather together under their gravity. They merge and become more concentrated.
• As it becomes denser, the temperature increases. The nuclei of hydrogen and other light elements fuse together. The core gets hotter and it shines.
• Our sun shines because of the hydrogen fusion at the core. This is the main stage of a stars life. Can continue for billions of years.
• Radiation flows out of the star steadily. This prevents it from collapsing in on itself. This stays balanced until all the hydrogen fuses in the core.

Death:

• When the star runs out of hydrogen, it expands. It cools down and becomes a red giant.
• Helium and other elements fuse to form heavier elements.
• When there is no more elemetns to fuse, fusion stops and the star collapses in on itself, becoming a white dwarf. This is a small, hot and dense star. They eventually go cold. Red->Yellow->White.
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## Supernovs and black holes

Bigger stars collapse and reach the white dwarf stage. Then in a dramatic explosion called a supernova.

This compresses the core into a neutron star, made from neutrons. If it has a large enough mass it will become a black hole. The gravity of a black hole means nothing can escape it, it is so strong.

Low mass star: protostar > main stage > red giant > white dwarf > black dwarf

High mass star: protostar > main stage > red giant > white dwarf > supernove > neutron star > black hole if mass sufficient

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nice, brief summary :)

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is this all you need

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is this all of p3

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Great notes for p3 :)

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