Year 8 Revision notes Term 1, 2016. Physics

You will be allowed to use a calculator in the Physics exam.

Speed.

formula: speed = distance/time 

units m/s (metres per second)  or km/h (kilometres per hour)

You will need to be able to calculate speeds, compare speeds, e.g. which is faster, 60m/s or 120 km/h?

60m/s = 60 x 60m/minute = 60x60x60 m/hour.  Divide by 1,000 (1,000 m in 1 km) = 216 km/h

If you can remember 3.6,  multiply m/s by 3.6 to get km/h

Measuring speed: Reaction time, 0.2 seconds, is the delay between viewing and action, e.g. pressing a button on a stopwatch.  Not very accurate.

For accurate readings we should use electronic timers and light gates.

Distance/time graphs

These show how distance changes as time goes on.

You will need to explain what is happening in a distance/time graph.

From graphs, we can learn 2 things:

a) The gradient or slope of the graph = rise over run, or y/x

                                                         
b) The AREA under the graph. y times x.

                                                           
rise/run = gradient.

For a distance/time graph, gradient is rise/run = y/x =  m/s = units for speed.

The area under a distance /time graph, y times x =  m x s  (ms) tells us nothing.

Straight lines mean constant speed.

A flat line, or zero gradient, distance is not changing = stopped, at rest or not moving.

Changing speed

When speed changes with time, an object gets faster, or slower. Changing speed is called ACCELERATION and can be positive, faster, or negative, slower.

In this graph, speed is increasing. 

The gradient or slope shows changing speed = acceleration.    Gradient = rise over run, = y/x 

                                                                              Units: speed/time = m/s divided by s = m/s/s or metres per second per second, or metres per second squared.

What about the area under a speed/time graph?

Area = the area of the triangle formed by the graph.  1/2 height x base.

Units:  m/s x s = m,  metres.  The unit for distance.

Area under speed/time graph = distance traveled.

You should be able to do simple calculations for journeys, such as acceleration between 2 points, and distance travelled using speed/time graphs.

Formula for acceleration.        v is speed after, u is speed before     (u before v  in the alphabet)  and t is time.




  Using this formula is easy, just substitute what we know and do      the maths!

Sound

Anything that VIBRATES makes a sound. But, the sound needs something to travel in, a MEDIUM, which can be a solid, liquid or a gas.

Sound travels as a wave, but not quite like water waves. A vibrating object pushes and pulls particles in the medium around it. This squeezes and stretches the medium, which then makes the particles nearby do the same thing.
The sound then spreads out in all directions as a compression wave, or LONGITUDINAL WAVE.

A sound wave can be shown on an OSCILLOSCOPE as a wave, as above, but really it is showing high pressure (squeezed particles) as the top of the wave, and low pressure (stretched particles) as the bottom of the wave.

You need to remember these things about waves:

The number of waves passing a point in 1 second of time is called FREQUENCY. Units: hertz, Hz.

Wavelength and frequency tell us the pitch or note of the sound. Long wavelengths and low frequencies are low sounds, high frequencies and short wavelengths are high sounds.

Humans can hear frequencies of up to 20,000 Hz, but animals such as cats, dogs, bats and dolphins can hear much higher sounds.

Amplitude tells us the intensity of the sound, or how loud it is.  High amplitudes, loud, low amplitudes soft sounds.

The intensity of sound is measured in decibels, dB.  30 dB sound is 10 times louder than a 20 dB sound.  The safe level for sounds heard for long periods is about 80 to 90 dB.  Louder than that we should protect our ears with Ear Defenders of Ear Plugs.

Speed of Sound

In air, sound travels at about 330 m/s, faster if the air is hot and humid.

In water, about 1,500 m/s because particles in water are close together.

Much faster in solid objects like concrete or steel.

Musical Instruments

Different instruments can play the same notes, but they sound different. A piano sounds different to a violin.

This difference in sound is called TIMBRE.

Below are oscilloscope screens showing the same note played by a piano, guitar and violin in our music room.

 Piano

Violin

Guitar

Echolocation

When a sound wave hits an object, some of the sound is reflected back as an ECHO.

This idea is used by animals to 'see' things in the dark or in muddy water.

Bats and dolphins have developed this to a fine art.

Humans make use of echoes as fish-finders, SONAR in submarines and ships, and in Ultrasound machines in hospitals.

Light

Light isn't energy, but the transfer of energy from one place to the next.

Light is produced by LUMINOUS objects: Sun, stars, lamps, LCD screens, LED's, electrical sparks, and even BIOLUMINESCENCE in fireflies, some fish and even fungi.

Light travels in straight lines. We know this because when light is blocked, a shadow is cast. The name for a shadow is UMBRA, from umbrella. Half shadow, caused by light dispersion is a PENUMBRA.

Light travelling in straight lines explains how a pinhole camera works.

The thing the camera is pointed at is called the OBJECT. The picture on the screen at the back of the camera is called the IMAGE.

Because light travels in straight lines the image is upside-down, INVERTED

Seeing things

Light from luminous objects is EMITTED (given out)  and passes straight into our eyes, like the camera.

So how do we see non-luminous objects, like tables, cats and dogs?

When light hits an object some light is ABSORBED. But some light also bounces off. It is REFLECTED. This reflected light then enters our eyes.

If light passes straight through something, like air, clear water of clear glass it is TRANSMITTED.

Whether light is transmitted, reflected or absorbed depends on an object's material, what it is made of.

You must remember these terms.

Object:

Light is transmitted - object is transparent - water, air, glass

Light is transmitted but scattered in all directions - object is translucent - paper, ice, muddy water

Light is reflected or absorbed - object is opaque - wood, metal, concrete

Know how to label a diagram of the eye.

Speed of Light

The speed of light is about 300 million m/s, or 300,000 km/s.

Even at this speed, distances in space are huge. We measure these huge distances in terms of LIGHT YEARS l.y., the distance light travels in a year. About 9 trillion km.

Nearest star: 4.1 l.y.

Milky Way galaxy: 100,000 l.y. across

Andromeda galaxy, our sister galaxy: 2.5 million l.y. away.

Reflection

Ray Diagrams. Know how to draw ray diagrams to explain why things happen.

Reflection in a plane (flat) mirror.

The object in a plane mirror is: Upright, the same size, the same distance from the mirror and VIRTUAL (not real).

Law of Reflection

i is the angle of incidence - angle of the incident or incoming light ray with the normal line.

r is the angle of reflection - angle of the reflected ray with the normal line. (red line)

i = r

Refraction

When light enters water from the air, it appears to bend. This bending of light is called refraction.

Ray diagrams explain what happens.

How much light is refracted depends on the medium the light leaves and on the medium

the light enters.

Why does refraction happen?

Light travels as waves, a bit like waves on a pond.

When a light wave enters a medium such as glass, it slows down (or at least appears to).

If one end of a wave enters the glass first, it will move slower than the other end and pull the wave around. When all the wave is in the glass it moves at the same, slower speed in a straight line again.

The same thing will happen in reverse when the wave leaves the glass. One end will speed up first.

The RATIO of the two speeds in, say, air and water, or air and glass, is called REFRACTIVE INDEX, R.I.

Given the speed of light in air is almost the same as in a vacuum, we can use RI to work out the speed of light in a given medium.

e.g. if light travels from air to water, and RI water is 3/2

RI = Speed of light in air                       3/2 = 300 million m/s                

        Speed of light in water                             speed of light in water

so, speed of light in water is 200 million m/s

note.

Light doesn't really slow down in water or glass. The light is being absorbed and emitted by particles in the medium. This takes a bit of time. Between particles the light travels at its normal speed, 300 million m/s.

More particles, or different particles, this happens more often or takes longer.

This explains why light speeds up again when it leaves the medium and enters air again.