# P5

Satellites, Gravity and Orbits, Speed and Velocity, Combining Velocities and Forces, Equations of motion, Projectile motion, Momentum, Radio waves and Communication, Interference of waves, diffraction patterns and polaristion, Refraction, Two Special Cases, Refractive Index and Snell's Law, Images and Converging Lenses, Ray Diagrams and Uses-Magnification and Cameras.

HideShow resource information
• Created by: Amina
• Created on: 21-04-12 15:51

## Satellites

Satellites are set up for many purposes:

Monitoring weather and climate          Communications                                             Space Research                          Spying                      Navigation

Communications satellites are put in a high orbit over the equator. The orbit takes 24 hours to complete. They stay about the same point on the earth's surface because the earth rotates with them = Geostationary. They are ideal for telephone and TV because they're always in the same place and can transfer signals very fast.

Weather and Spying satellites are in a low polar orbit. The satellites sweep over both poles in just a few hours whilst the earth rotates beneath it. They are closer to the earth than geostationary satellites so the pull of gravity is stronger so they move much faster.

1 of 15

## Gravity and Orbits

Gravity provides the centripetal force that causes orbits. An orbit is a balance between the forward motion of the object and the force pulling it towards (the centripetal force). The planets move around the sun in an almost circular orbit. The centripetal force that makes it happen is provided by gravity between each planet and the sun.

The closer you get to a star or planet, the stronger the force of attraction. Because of this, planets near the sun move faster and orbit quicker.

Comets orbit the sun but have very eccentric orbits. The sun isn't at the centre of the orbit but near one end, so their orbits take them a long way from the sun then back in close again. The comet travels much faster when its nearer to the sun. This is because the increased pull of gravity makes it speed up the closer it gets to the sun.

2 of 15

## Speed and Velocity

Speed is a scalar quantity e.g mass temperate time length

Velocity is a vector quantity (it has a direction as well) e.g force, displacement, acceleration, momentum

Relative Speed compares the speed of two different objects

A car going the same way as you will only have a small speed relative to your car

A car going the opposite way will have a bigger speed relative to you

3 of 15

## Combining Velocities and Forces

Across the current

A boat is going west at 14 m/s in a river with a current running north at 8 m/s. Resultant Velocity = ?

Draw the right angled triangle. Use Pythagoras theorem:

√8­2142 = 16.1 m/s

4 of 15

## Equations of Motion

Equations of Motion

u = initial velocity

v= final velocity

s= displacement

t=time

a=acceleration

5 of 15

## Projectile Motion

A projectile is something that is projected/dropped and then has only gravity acting on it.   The path a projectile takes through the air is called its trajectory. It is always the shape of a parabola.

For something that starts of horizontally there is constant horizontal velocity because there are no horizontal force. Vertical velocity increases steadily as gravity accelerates it downwards.

6 of 15

## Momentum

Momentum= Mass x Velocity

More mass and more velocity = more momentum the object has.

Momentum has a size and direction (vector)

Momentum before = Momentum After (momentum is conserved)

Forces cause changes in Momentum:

Force acting = change in momentum/time taken for change to happen

A larger force means faster change of momentum...Cars are designed to slow people down over a longer time when they have a crash - longer it takes for change in momentum, the smaller the force:

CRUMPLE ZONES increase time SEAT BELTS increase time by stretching

7 of 15

Ground waves: travel in close contact with the ground as they spread out from the transmitter. Used by LWMW radio bands (up to 3 MHz).

Sky waves: Frequencies up to about 30Mhz can reflect off a layer of the atmosphere called the ionosphere. This allows the wave to travel longer distances and deals with the curvature of the earth.

Space waves: Microwave signals have a high frequency - over 3000 MHz for satellite TV and telephone. They pass easily through the atmosphere and reflect off satellites orbiting the earth enabling the signal to reach distant parts of the planet.

All waves diffract when they pass through a narrow gap or past an object. The longer the wavelength of a wave, the more it will diffract.Smaller gap = waves get diffracted more.

Long wavelength radio waves have a really long range. They spread out in all directions so are great for broadcasting and can diffract over hills.

8 of 15

## Interference of Waves & Diffraction Patterns & Pol

All waves cause some kind of disturbance in a medium. When two waves meet at a point they both try to cause their own disturbance. Waves either disturb in the same direction (constructive interference) or in opposite directions (destructive interference).

When light diffracts through a gap you get patterns of light and dark.There is a bright central fringe with alternating dark and bright fringes on either side of it.

EM waves are transverse:

Light and all EM waves    Ripples on water

Transverse waves can be plane polarised:

Ordinary light waves are a mixture of vibrations in different directions. Passing light through a polarising filter transmits vibrations only in one direction. That means if you have two polarising filters at right angles to each other, no light can get through.

9 of 15

## Refraction

Refraction is when waves change direction as they enter a different medium. This is caused by the change in speed of the waves. The speed change also causes the wavelength to change. The frequency stays the same.

Refraction is shown by Waves in a ripple tank slowing down. The waves travel slower in shallower water causing refraction. There's a change in direction and a change in wavelength too.

Refraction of light - The ray bends towards normal as it enters the denser medium and away from the normal as it emerges into the less dense medium.

Refraction is always caused by the waves changing speed

The ratio of speed of light in a vacuum to the speed of light in a medium is called the refractive index of the medium. The higher the refractive index, the more the light bends when it enters or leaves the medium.

10 of 15

## Refraction: Two Special Cases

Different colours of light are refracted by different amounts. This is because they travel at slightly different speeds in any given medium.

A prism can be used to make the different colours of white light emerge at different angles.

Red light is refracted the least and violet the most.

Refractive Index= speed of light in vacuum/speed of light in that material

Light slows down in glass so the refractive index of glass is high. The refractive index of water is a bit lower so light doesn't slow down as much in water.

11 of 15

## Snell's Law

Snell's law says that when an incident ray passes into a material: n=sin i/sin r

Total internal reflection:

When light leaves a material with higher refractive index and enters a material with a lower refractive index, it speeds up and so bends away from the normal.If you keep increasing the angle of incidence the angle of refraction gets closer and closer to 90 degrees. Eventually it reaches a critical angle C for which r=90 degrees. You get total internal reflection - no light leaves the medium.

You can find the critical angle C:

Sin C= nr/ni

Higher refractive index, the lower the critical angle.

12 of 15

## Images and Converging Lenses

Describing an image:

How big it is compared to the object     Whether its upright or inverted                                  Whether its real or virtual        Where it is in relation to the focal points

Converging Lenses focus light:

A converging lens is convex. it causes rays of light to converge to a focus. If the rays entering the lens are parallel to each other and to the axis, it focuses them at a point called a focal point. Converging lenses can make real or virtual images depending on how close the object is to the lens.

13 of 15

## Ray Diagrams

Distance from the lens affect the image:

An object at 2F will produce a real upside down image the same size as the object at 2F. Between F and 2F it'll make a real upside down image bigger than the object and beyond 2F. An object nearer than F will make a virtual image the right way up, bigger than the object and same side of the lens.

14 of 15

## Uses - Magnification and Cameras

Magnifying glasses use convex lenses. They produce a virtual image which you cant project on a screen.

Magnification Formula:

Magnification= Image height/Object height

When taking a photograph of a flower, the light from the object travels to the camera and is refracted by the lens, forming an image on the film. The image on the film is real because light rays actually meet there. The image is smaller than the object because the object is further away than the focal length of the lens. The image is inverted. The same thing happens in our eye. A real inverted image forms on the retina Our brains flips the image the right way.

15 of 15