# Physics P3

Revision cards covering the raw skeleton of the AQA physics P3 units.

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• Created by: melissa
• Created on: 08-06-09 09:21

## Moments and stability

Moment = Turning effect of force.

• Force acts at right angles from spanner for maximum moment)

Moment (Nm) = Force (N) X Perpendicular distance from pivot (M)

Think of a seasaw.... If the seasaw is balanced - centre of mass is above pivot and..

Total clockwise moments = total anticlockwise moments

Otherwise the body will topple due to a resultant force

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## Balance and circular motion

Centre of mass = the point where the mass of the body is thought to be concentrated

To be most stable... Wide Base + low centre of mass

• Topples when centre of mass moves outside of base
• If weight doesn't act on line of pivot causes a resultant moment.

Circular Motion

• If moving on circular path muct constantly be unbalanced forces acting
• Constantly changing direction and accelerating
• Forces act towards centre of circle
• Force keeping it moving in a circle = CENTRIPETAL FORCE
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## Centripetal force

Centripetal force needed increases when:

• Mass increases
• Speed increases

Vehicle on roundabout- centripetal force due to friction between tyres and road

Circling aircraft- centripetal force due to combined effect of weight and lift acting upon it. centripetal force = result of these two forces

In planets and satellites centripedal force is due to gravity

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## Gravitational attraction

Gravity increases when:

• The greater the mass of two bodies
• Distance is closer between the two bodies

Orbit of planet is an ellipse with sun at focus

To remain in oribt a particular speed is need otherwise it will spiral into sun or fly off its orbit.

Orbits (of satellites)..

Geostationary- Communications, 24h orbit, high, equator

Low polar- weather, spying, over both poles, hours orbit

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## Images and light

Mirrors and lenses form images

Normal is a perpendicular line from surface at the point of incident.

• Normal bisects angle of reflected light
• Angle of incidence = Angle of reflection

In a plane mirror...

Object is as far behind mirror as object is in front

In a convex mirror- image is always...

• Virtual
• Upright
• Smaller than object

Further object from mirror, the smaller the image

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## Refraction

REFRACTION = change of direction of light rays

Convex lens- Converging

• Pass through focal point on other side (this ray was parralel to the axis)
• Ray passing through centre of lens continues in same direction

Concave lens- Diverging

• Incident ray parallel to axis travels through focal point
• Travelling through centre contiues in same direction
• Image is always -Virtual
• Right way up
• Smaller than object
• On same side of lens as object

Magnification(converging lense) must be closer to lens than Focal Point

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## Sound

Sound...:

• Is caused by vibrations
• LONGITUDINAL WAVES
• Cannot travel through a vacuum
• can reflect and refract
• human 20-20,000 Hz (beyound 20,000 is ULTRASOUND)
• pitch increase with frequency
• loundness increases with amplitude

Ultrasound:

Uses: Industry cleaning, quality control, medicine, prenatal scanning

• High frequencies
• Partically reflected an media boundaries (can detect distance)
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## Sound equations

Frequency (Hz) = . 1 .

Time period (s)

Speed (m/s) = Frequency (Hz) X wavelength (m)

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## Motors

Electirc currents create magnetic fields

MAGNETIC FEILD: region where magnetic materials and wires experience a force acting upon them

These forces can make things move (MOTOR EFFECT)

Electric current + magnetic field = kinetic energy

Force increased by:...

• Increasing magnet field strength
• Increasing size of current

(if either of the above are reversed sop is the direction of force)

Force on one side is in opposite direction to force on the other side

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## generator

Kinetic energy + Magnetic field = Electricle current

• Coil rotated in magnetic field
• as spins a current is induced in the coil
• this changes direction every half turn
• Produce AC voltage
• faster revs produce higher voltage

Direction of induced P.d. is reversed when either direction of motion or polarity of magnet is reversed

Induced P.d. increases when:

• Speed of movement increases
• strength of magnetic field increases
• number of turns on the coil increases
• area of coil is greater
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## Transformers

Step up (increase P.d.) or Step down (decrease P.d.)

More turns of Secondary coil More turns on primary coil

Must be A.C. current

• A.C. current creates changing magnetic field in the iron core
• hence induces a change of P.d. across Secondary coil

Used in National Grid:

• to transmit a lot of power you need high current or high voltage
• Cheaper to boost voltage
• transformers needed
• Less heat energy lost
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## Stars

Gravitational forces formed galaxies of stars

Sun radiates energy due to hydrogen fusion in core

ALL ELEMENTS MADE ORGINALLY FROM HYDROGEN DUE TO NUCLEAR FUSION (elements are distributed from stars by supernova at end of life)

Older stars- all hydrogen in core turns to hellium which fues into even heavier elements

Masses atracted together after explosions to create stars and planets.

Low mass stars: (PMRWB)

PROTOSTAR --> MAIN STAGE --> RED GIANT --> WHITE DWARF--> BLACK DWARF

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High mass stars:(SAME UNTIL WHITE DWARF)

WHITE DWARF --> SUPERNOVA --> NEUTRON STAR ---> BLACK HOLE (sufficient mass)

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