# Physics Unit 3

Physics Unit 3

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## Moments

- A moment is the turning affect of a force.

- Even if the forces acting on a body are balanced in the same sense that they do not cause the body to change speed, they can still make the body turn.

Moment = Force x Perpendicular distance between line of action and pivot

(Nm)           (N)                                        (Metres)

E.g. force=10N, distance=0.3m = 10N x 0.3m = 3Nm

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1)To get the MAXIMUM MOMENT you need to push at RIGHT ANGLES to the spanner. 2) Pushing at any other angle means a smaller moment. 3) This is as the perpendicular distance between the line of action and pivot is smaller. 4) The centre of mass of a body is that point at which the mass of the body may be thought to be concentrated. 5) if suspended, a body will come to rest with its centre of mass directly below the point of suspension. 6) The centre of mass of a symmetrical body is along the axis of symmetry. 7) If a body is not turning, the total clockwise moment must be exactly balanced by the total anticlockwise moment about any axis. 8) If the line of action of the weight of a body lies outside the base of the body, there will be a resultant movement and the body will tend to topple.

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## Balance and Stability

1) If the total anticlockwise moments do not equal the total clockwise moments, there will be a resultant moment, so the object will turn.

2) Unstable objects tip over easily.

3) The position of the centre of mass is very important.

4) Most stable objects have a wide base and a low centre of mass.

5) An object will begin to tip over if its centre of mass moves beyond the edge of its base.

6) If the weight does not act in line with the pivot, it will cause a resultant moment.

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## Circular Motion

1) A body remains stationary, or keeps moving at the same speed in a straight line, unless an unbalanced force acts upon it. 2) If a body moves in a circular path there must constantly be an unbalanced force acting upon it. 3) Velocity is both speed and direction of an object. 4) If an object is traveling in a circle, is a constantly changing direction. 5) This means it must be accelerating. 6) This means that there must be a force acting on it. 7) This force acts towards the centre of the circle. 8) The force that keeps something moving in a circle is called a CENTRIPETAL FORCE.

The direction of the centripetal force is always towards the centre of the circle.  The centripetal force needed to make a body perform circular motion increases as: mass of the body increases, speed of the body increases, radius of the circle decreases.                                                                                                           The centripetal force one a vehicle moving round a roundabout is die to friction between the tyres and on the road. The centripetal force on an aircraft circling round is due to the combined effect of its weight and the lift force on it. The centripetal force is the resultant of these two forces.

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## Gravitational Attraction

1) The planets, like the Earth, orbit the Sun. 2) The Earth, Sun, Moon and all other bodies attract each other with a force. 3) This force is GRAVITY. 4) The bigger the masses of the bodies the bigger the force of gravity between them. 5) As the distance between the two bodies increases the force of gravity decreases. 6) The orbit of any planet is an eclipse with the Sun at one FOCUS. 7) Gravitational force provides the centripetal force. 8) This is what allows planets and satellites to maintain their circular orbits. 9) The further away an orbiting body is, the longer it takes to make a complete orbit. 10) To stay in orbit at a particular distance, smaller bodies, including planets and, must move at particular speed around larger bodies.

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ARTIFICIAL SATELLITES were sent up by humans for four main purposes: Monitoring of the Earth (weather and climate), Communications (phone and television), Space research (Hubble Space Telescope), Spying.

Two main kinds of orbit: GEOSTATIONARY ORBIT: Used for communications, High orbits, over the equator, Take exactly 24 hours to complete, Allows them to say above the same point, Ideal for telephone and television, Can transfer signals from one side of the Earth to another quickly.

LOW POLAR ORBIT: Used for weather and spying (i.e. monitoring), Satellite sweeps over both poles, Takes just a few hours to complete a full orbit, Each time a satellite comes round it can scan the next bit of the globe, This allows the whole surface to be monitored each day.

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## Images

1) A real image is where the light from an object comes together to form an image on a 'screen' - like the image formed on an eye's retina. 2) A virtual image is when the rays are diverging, so the light from the object appears to be coming from a completely different place. 3) When you look in a mirror you see a virtual image of your face - because the object (your face) appears to be behind the mirror. 4) You can get a virtual image when looking at an object through a magnifying lens - the virtual image looks bigger and further away than the object actually is.

Reflection: 1) Reflection of light is what allows us to see objects. Light bounces off them into our eyes. 2) When light reflects from an uneven surface, the light reflects off at all different angles and you get a diffuse reflection. 3) When the light reflects from an even surface, then it's all reflected at the same angle and you get a clear reflection.

Angle of INCIDENCE = Angle of REFLECTION

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## Mirrors

1) The image is the same size as the object in a plane mirror. 2) It is AS FAR BEHIND the mirror as the object is in front. 3) It is formed from diverging rays, which means it's a virtual image.

For a concave mirror: 1) Uniformly curved mirrors are like a round portion of a sphere. The centre of the sphere is the centre of curvature, C. 2) The centre of the mirror's surface is called the vertex. 3) Halfway between the centre of curvature and the vertex is the focal point, F. Rays parallel to the axis of a concave mirror reflect and meet at the focal point. 4) The centre of curvature, vertex and focal point all lie on a line down the middle of the mirror called the axis. 5) The centre of curvature and focal point are in front of a concave mirror, and behind a convex mirror.

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## Refraction

1) Refraction of light is when the waves change direction as they enter a different medium. 2) This is caused entirely by the change in speed of the waves. 3) That's what makes ponds look shallower than they are - light reflects off the bottom and speeds up when it leaves the water, making the bottom look like it's nearer than it is.

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## Lenses

Light is refracted when it enters and leaves glass prisms. 1) The ray bends towards the normal as it enters the denser medium, and away from the normal as it emerges into the less dense medium. Try to visualise the shape of a wiggle/zig zag - that can be easier than remembering the rule in words. 2) Note that different wavelengths of light refract by different amounts. So white light disperses into different colours as it enters a prism. A rectangular prism has parallel boundaries, so the rays bend one way as they enter, and then bend back again by the same amount as they leave - so white light emerges. But a triangular prism, the boundaries aren't parallel, which means the different wavelengths don't recombine, and you get a nice rainbow effect.

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## Lenses

There are two main types of lens:

Converging: 1) A converging lens is convex - it bulges outwards. It causes parallel rays of light to converge (move together) to a focus. 2) The focal point of a converging lens is where rays hitting the lens parallel to the axis all meet.

Diverging: 1) A diverging lens is concave - it caves inwards. It causes the parallel rays of light to diverge (spread out). 2) The focal point of a diverging lens is the point where rays hitting the lens parallel to the axis appear to all come from - you can trace them back until they all appear to meet up at a point behind the lens.

The axis of a lens is a line passing through the middle of the lens. Each lens has a focal point in front of the lens, and one behind.

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## Lenses

There are three rules for reaction in a converging lens: 1) An incident ray parallel to the axis refracts through the lens and passes through the focal point on the other side. 2) An incident ray passing through the focal point refracts through the lens and travels parallel to the axis. 3) An incident ray passing through the centre of the lens carries on in the same direction.

And Three riles for refraction in a diverging lens: 1) An incident ray parallel to the axis refracts through the lens, and travels in line with the focal point (so it appears to have come from the focal point). 2) An incident ray passing towards the focal point refracts through the lens and travels parallel to the axis. 3) An incident ray passing through the centre of the lens carries on in the same direction.

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## Magnification and Cameras

Magnifying Glasses use convex lenses. Magnifying glasses work by creating a magnified virtual image. 1) The object being magnified must be closer to the lens than the focal length. 2) The image produced is a virtual image. the light rays don't actually come from where the image appears to be. 3) Remember "you can't project a virtual image onto a screen" - that's a useful phrase to use in the exam if they ask you about virtual images.

Magnification formula: Magnification = Image height / Object height

this formula can be used to work out the magnification produced by a lens or a mirror at a given distance.

E.g. A coin with diameter 14mm is placed a certain distance behind a magnifying lens. The virtual image produced  has a diameter of 35mm. What is the magnification of the lens at this distance? Magnification = 35 / 14 = 2.5

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## Sound Waves

1) Sound waves are caused by vibrating objects. These mechanical vibrations are passed through the surrounding medium as a series of compressions. they're known as longitudinal waves. 2) sometimes the sounds will eventually reach someone's eardrum, at which point the person might hear it (if it's loud enough and in the right frequency). 3) Because sound waves are caused by vibrating particles, the denser the medium, the faster sound travels through it, generally speaking anyway. Sound generally travels faster in solids than in liquids, and faster in liquids than in gases.

Sounds waves can reflect and refract. 1)Sound waves will be reflected by hard flat surfaces. Things like carpets and curtains act as absorbing surfaces which will absorb sounds rather than reflect them. 2) This is a very noticeable in an empty room. A big empty room sounds completely different once you've put carpet and curtains in, and a bit of furniture, because these things absorb the sound quickly and stop it echoing around the room. 3) Sound waves will also refract (change direction) as they enter different media. As they enter denser material, they speed up.

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We hear sounds in the range 20 - 20000 Hz. 1) The frequency of a wave (in Hz) is the number of waves in 1 second. 2) The human ear is capable of hearing sounds with frequencies between 20 Hz and 20000 Hz.

Sound does not travel in a vacuum. 1) Sound waves are transmitted by vibrating particles - so they can't travel through a vacuum. (No particles, see.) 2) This is nicely demonstrated by the bell jar experiment. 3) Air is sucked out by the vacuum pump and the sound gets quieter until there is no sound. 4) The bell has to be mounted on something like foam otherwise you'd hear the vibrations it makes on the solid surface instead.

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## Ultrasound

Ultrasound is sound with a higher frequency than we can hear. Electrical devices can be made which produce electrical oscillations of any frequency. These can easily be converted into mechanical vibrations to produce sound waves beyond the range of human hearing. This is called ultrasound and it has loads of uses.

Ultrasound waves get partially reflected at a boundary between media. 1) When a wave passes from one medium into another, some of the wave is reflected off the boundary between the two media, and some is transmitted (and refracted). This is partial reflection. 2) What this means is that you can point a pulse of ultrasound at an object, and wherever there are boundaries between one substance and another, some of the ultrasound gets reflected back. 3) The time it takes for the reflections to reach a detector can be used to measure how far away the boundary is. 4) This is how ultrasound imaging works.

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