# GCSE Physics Unit 3

Basically, anyone who's taking GCSE physics unit 3 AQA (but I think you will be able to use them whatever board you're on)

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## Introduction and turning effect of moments

I will put the answers to all the questions which are on these cards on one (or two) last card(s) so you can't cheat!!!

I will also try and add colour where I can to try and make it look more interesting, so important words will be highlighted.

Moments:

Some questions first...

1. The turning effect (or.........) on a spanner depends upon the ......... applied and on the ......... from the nut to the force.

2. The centre of mass (centre of ........) of a metre rule is at the ...... of the rule.

3. An object balances at its centre of ............

Revise moments on the next card...

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## Turning effect of moments

Finding the centre of mass of a card: A suspended object will come to rest with its centre of mass below the pivot. This is so the weight doesn't cause a turning effect (moment) Hanging the card from a pivot in two diffrent places with a piece of string with a weight on where the pivot is. (which you draw a line down) allows you to find the centre of mass.

Stable and unstable objects: If the line of actionof the weight lies outside the base of the object, there is a moment, and the object tends to fall over. The best example of this is two glasses, one which looks like \_/ and another one which looks (sort of) like this |_| Which is the most stable? The line of action is more easily pushed outside of the base on the \_/ glass so this is less stable than the |_| glass.

More on next card...

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## Turning effect of moments continued

You get all formulae that you need given in the exam, but it's good to know them anyway. The formula for moments is

moment (Nm, newtonmetre) = force (N, newton) x perpendicluar distance (from the line of action to pivot) (m, metre)

The law of moments: If an object is not turning, the moments must be balanced, thus: In equilibrium; TOTAL anticlockwise moment = TOTAL clockwise moment

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## Moving in a circle

If you tie a weight to some string and spin it in a circle above your head (I am not suggesting you do this, but you can if you want...just make sure you don't break anything! I am not responsible for any breakages) it makes a circular orbit. However if the string breaks, the object would move in a straight line...to move in a circle there must be a resultant force on it (the tension in the string).

An object moving in a circle at constant speed is changing its direction all the time. This therefore means that its velocity is changing all the time (because velocity is a vector quantity, with size and direction! Don't forget!) Because of this, it is always accelerating towards the centre of the circle. So it can accelerate, there must be a resultant force on it. This is the centripetal force acting towards the centre of the circle, which can be provided by a string or by friction or by gravitational attraction or other forces...

Some examples are: The object whirling in a circle - tension in the string, a car turning a corner - friction between tyres and road, you on car seat as it turns corner - friction between you and the seat, the Earth orbiting the sun - gravitational attraction of the Sun on the Earth, an electron orbiting an atom - attraction between charges (negative electron and positive nucleus)

More on next page...

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## Moving in a circle

The centripetal force will need to be greater if the mass of the object is greater, the speed of the object is greater or the radius of the circle is smaller.

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## Planets and satellites

Some questions first:

1. The Moon orbits the Earth once every ........... . It is held in its ............ by the .......... pull of the .......... .

2. The Earth orbits the Sun once each ...... (...... days), held by the ....................... pull.

3. The planets move in ........... around the ........... . We see the planets because they ......... light from the .......... .

4. ......... can be put in orbit round the Earth. They are held in orbit by its ........... pull.

Revise Planets and Satellites on the next card...

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## Planets and satellites

Gravitational force:

The Earth, the Sun, the Moon and all other bodies attract each other, with a force called gravity. The pull of gravity on you, by the earth, is called your weight.

The bigger the masses of the bodies, the bigger the force of gravity between them.

As the distance between 2 bodies increases, the force of gravity decreases. At twice the distance it is only 1/4 as much.

The gravitational force provides the centripetal force needed to keep planets, moons and satellites in their (nearly) circular orbits...these are called elliptical orbits.

More on next page about planets and satellites...

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## Planets and satellites

Orbits of planets and satellites

The planets are held in orbit around the Sun, by the gravitational pull of the Sun. Their orbits are slightly squashed circles (ellipses) with the Sun quite close to the centre.

To stay in orbit at a particular distance, a planet must move at a particular speed round the Sun. The futher away a planet is, the longer it takes to make a complete orbit (eg Pluto [which is no longer a planet] takes 248 years to orbit)

Comets are also held in orbit by the gravitational pull of the Sun. They have very elliptical orbits, so sometimes they are near the Sun and sometimes far out in the Solar System (sometimes further than Pluto!)

More on next card...

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## Planets and Satellites

Satellites and gravity

Satellites can be launched to orbit the Earth. They stay in orbit because of the combination of the high speed and the force of gravity of the Earth.

To stay in a particular orbit, they have to travel at the right speed:

The higher the satellite, the slower the speed and the longer it takes for one orbit.

Uses of satellites

Monitoring or observation satellites eg weather, military (spy)

Communications satellites eg telephones, TV

Astonomical satellites eg space telescopes taking pictures outside the Earth's atmosphere

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## Planets and satellites

Satellite orbits

Satellites can be launched into polar orbit or equatorial orbit.

Monitoring satellites are usually put into low polar orbit, so that as the Earth spins beneath them they can scan different parts of the Earth.

Communication satellites are usually put into a high orbit above the equator.

The height is adjusted so that the satellite moves round at exactly the same rate as the Earth spins ie. once every 24 hours. This means that the satellite is always in the same position when viewed from Earth. This is a geostationary orbit.

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## Mirrors and lenses

This topic is going to be very difficult to explain without diagrams, so if you're still not sure, look at a textbook/revision guide/your notes (I know, I hate doing that too!)

Some questions first, again!

1. At any mirror, the angle of incidence is .............. to the angle of ................. .

2. When light travels into a glass block, the rays are bent (or ......................) ............ the normal line.

Revise mirrors and lenses on the next few cards...

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## Mirrors and lenses

Reflection

The image formed in a plane mirror is:

• The same size as the object,
• Upright (or erect!)
• laterally inverted (it's the other way round)
• Virtual (you can't show it on a screen)

Refraction:

When light enters something that is "optically more dense" than air (like a glass block or water) the ray of light bends or refracts towards the normal line. When white light enters into a prism, however, the spectrum is formed (Red, Orange, Yellow, Green, Blue, Indigo, Violet) There are easy ways to remember this order and one is Richard Of York Gave Battle In Vain.

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## Mirrors and lenses

Curved mirrors

A concave mirror converges rays - logical, huh?! Well, sort of...

The image which is formed by a concave mirror depends on where the object actually is. (You will need to see a diagram to understand all this, really, and as I can't show one, I'm afraid you'll have to try elsewhere!)

A convex mirror diverges rays.

The image is always

• Virtual
• Diminished (or smaller)
• Upright (or erect!)
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## Mirrors and lenses

Lenses

The image formed by a converging lens depends on where the object is. (Once again, you will need to see a diagram to understand all this, really, and as I can't show one, I'm afraid you'll have to try elsewhere!)

The image formed by a diverging lens is always

• Virtual
• Diminished (or smaller)
• Upright (or erect!)

Images in mirrors and lenses can be real or virtual, inverted or upright, magnified (enlargened) or diminished

Magnification = height of image I divided by height of object O

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## Sound

As always, some questions to start...

1. All ............. are caused by vibrations.

2. The speed of sound in air is about 320 ......... per second

3. Like any other waves, a sound wave transfers ............. without transferring any matter.

4. Echoes are due to the .......... of sound.

5. Sound can travel through solids, .................. and ................ . It cannot travel through a ..............., and therefore .................... .

6. A sound wave is a ............. wave. The .............. is transferred from molecule to ....................... .

Revise sound on the next few cards...

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## Sound

Using an oscilloscope

The wave form of a sound can be displayed using a microphone and an oscillopscope. When the longitudinal sound wave hits the microphone, it transmits through the microphone into the oscilloscope. We get what is called a "transverse wave" on the screen. As I can't show diagrams, use your textbook/notes/google images to see what they look like.

Loudness and amplitude

The greater the amplitude of the vibrations, the louder the sound (the number of waves stay the same, but they are stretched vertically)

Pitch and frequency

The number of complete vibrations per second is called the frequency. It is measured in Hertz (Hz) The higher the frequency, the higher the pitch of the sound. A low note has a low frequency whereas a high pitched note has a high frequency.

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## Sound

Quality of the sound

Different musical instruments sound different even if they play the same note. They have different tones or quality. This will depend on the waveform of the sound. (Once again, I can't show diagrams, so look in your textbook or something)

Ultrasonic waves (ultrasound)

The normal range for human hearing is 20Hz to 20,000Hz. Sound waves with a higher frequency than this are called ultrasonic waves. Because their freqency is so high, they have a very short wavelength.

Uses of Ultrasound

They can be used in industry for quality control eg detecting flaws in metal, in medicene eg for pre natal scanning, in industry for cleaning dirty objects eg watches, street lamp covers

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