interesting physics o level

8/17/2019 Interesting Physics o Level

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A Fish's View of the World

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Though fish live under water, their view is not restricted to seeing underwater. Many fish in fact

need to be able to see insects and other animals above the water as they are sources of food or potential threats to the fish. Fish see everything above the water, distorted a little, and possibly a

lot if the surface is rippled.. Their field of view is restricted by the angle at which total internal

reflection takes place.

If the refractive index of water is n then the critical angle is given by

The fish can also see obects under the water, either directly or by reflection from the watersurface.

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Acoustics

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The reverberation of sound is particularly noticeable in cathedrals and other large buildingswhere multiple reflections can occur rom the walls, roof and floor. In !t Paul"s #athedral in

$ondon it can take about % seconds for the notes of the organ to die away after the organ has

finished playing.

&xcessive reverberation in a concert hall is undesirable as it causes music and speech to soundconfused and indistinct. 'n the other hand it is not a good idea to have no reverberation at all.

!peakers and singers who have practised in an empty hall are often disconcerted by the seeming

weakness of their voices when they perform in the same hall full of people. The soft clothing of

the audience absorb the sound instead of reflecting it making the music and voices appearweaker. !ome degree of reverberation is desirable as it prevents a hall from being acoustically

dead and improve the sound in all parts of the building.

The most important acoustic property of a building is it"s reverberation time, defined as the time

taken for sound of a specified intensity to die away until it ust becomes inaudible. Thereverberation time depends on the volume and surface area of the hall, and on the sound

reflecting properties of the material covering the surfaces. ( reverberation time of between ) and

* seconds is usually found to be optimal

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For certain special purposes, it is necessary to have sounds in which surfaces completely absorb

all sound falling on them. The surfaces are lined with glass fibre or plastic fibre covered in

muslin. The management of sound is important in blocks of flats. In modern buildings the space between floors and ceilings are usually filled with some inelastic material which absorbs the

sound instead of transmitting it.

Air Bags, Seat Belts and Crumple Zones

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#ar crashes dont kill people + F'#&! -I$$ P&'P$&. People die in car crashes because impact

is too often instant, and the force of impact too large. If the time of impact can be prolonged,

then the acceleration and the force of impact can be reduced. This is because of ewton"s * nd

$aw of Motion, /)0.

1e can use the e2uation in /)0 to get If we increase the time of impact, we

can reduce the average force of impact and this will result in inuries that are less severe. Thereare three main ways to do this3

#rumple 4ones3 These are semi rigid structures a the front and back of the vehicle, which are

designed to absorb energy and collapse slowly on impact.

!eat 5elts are the secondary line of defence. They are designed to stop passengers becoming

proectiles inside the car by restraining them in their seats. If rear passengers were not restrained

they might kill passengers in the front seat by impacting them from behind, as well as seriouslyinuring themselves. If front seat passengers were not restrained they could impact the

dashboard6steering wheel6windscreen, which are all rigid and will cause serious inury.

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The final line of defence is air bags. They are designed to inflate if subect to a force above a

certain level. They then absorb as much of the kinetic energy of impact as possible when they are

hit by a person. 7as is very compressible and very well suited to absorbing the energy of impact.


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Air Lubrication

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'il is a lubricant, used to reduce friction in car engines. 1ater also acts as a lubricant + try

walking down the street in leather soled shoes when it is raining + don"t carry anything you don"twant to break should you fall over.

(ir too has long been thought of as a possible lubricant. (s long ago as )89: the possibility was

suggested, by a French scientist, 7ustav ;irn. 1hen a fine surface is re2uired it is usual to use agrinding wheel. ot the slightest wobble may be present, else the surface would not be smooth.

'il lubricated bearings, with very tight clearances had to be used because oil is virtually

incompressible, so it was not possible to compress the oil between any bearings in the machinery

to take up any imperfections in the bearings.

(ir though, is much more compressible.


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The three main types of nuclear radiation are alpha, beta and gamma radiation. Their properties

are summarised in the following table.

Type

Mass in units of

atomic mass units

#harge

?escription

(lpha

:

*

#onsists of * protons particles and *

neutrons

5eta

>

+)

( fast movingelectron

7amma

>

>

;igh energy

electromagnetic

radiation

(ll nuclear radiation originates from the nucleus. 1hen the nucleus undergoes decay, it may emit

one of these types of radiation which generally result in a more stable nucleus. ( nucleus mayundergo several decays before it becomes completely stable.

"roperties

(lpha radiation is the least penetrating. In a cloud chamber, it leaves a dense, heavy track. It

may be stopped by a piece of paper of a few cm of air. 5ecause it is charged it will curve in an

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electric or magnetic field. It is the most dangerous if ingesting since all the radiation will then be

absorbed by the body.

5eta radiation is more penetrating. 5eing lighter than an alpha particle, it leaves a less visible,longer track. It may be stopped by a few cm of aluminium. 5ecause it is charged it will curve in

an electric or magnetic field, and being oppositely charged to an alpha particle, it will curve theopposite way. It is less energetic and more penetrating than alpha radiation so less dangerous if

ingested since more will escape from the body.

7amma radiation is very penetrating. It can only be stopped by several cm of lead. It leave a very

wispy track in a cloud chamber, and is not charged, so does not curve in an electric field. Most

will escape from the body if ingested so this is the least dangerous form of radiation.

#ses

(lpha radiation is used in fire alarms. If smoke enters the detector, the amount of radiation

leaving the sensor will fall significantly and the alarm will be triggered.

5eta radiation is used to control the thickness of sheet metal. If the metal is too thick the amountof radiation reaching the sensor will be reduced and the rollers will make the metal thinner via a

feedback mechanism.

7amma radiation is used in cancer treatment. It preferentially kills rapidly dividing cancer cells

over healthy cells. ?oses over several weeks may reduce the si@e of tumours significantly in thisway.

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