# P5 - Space For Reflection

Physics GCSE, OCR Gateway, P5

## P5a - Satellites, Gravity and Circular motion

Gravity - A force which acts on all masses. The bigger the mass the bigger the force. The further apart the masses the smaller the force.

Period - The time taken to complete on orbit. Comits have very elliptical orbits around the sun. When the commit is further away it slows down as the gravitational force is less.

The lower the orbit the: Shorter the period of the orbit; stronger the gravitational force; greater the speed

Geostationary satellites - Are in geostationary orbits - Used for communications and weather forecasting. These satellites all orbit: above the equator; witha period of 24 hours; at a hright of 35,800; above a fixed position on the earth.

Low polar orbits - Pass over the earths poles andh have lower orbits then that of geostationary satellites. The orbit period depends on the height of the orbit above the earth. They can be used for: weather forecasting; monitoring climate change and spying.

**G**oblins **H**ave **P**eculiar **L**egs - **G**eostationary **H**igh, **P**olar **L**ow

## P5b - Vectors and Equations of Motion

Scalars - quantities which have magnitude (size) only. (Distance, speed, energy)

Vectors - quantities which have magnitude and associated direction. (displacement, velocity, acceleration) Velocity is a speed in a given direction.

Resultant - one or more vectors acting together which can be replaced by a single vector which carries the same effect. tanX = Va/Vb = Resultant and Vr^2 = Va^2 + Vb^2

## P5c - Projectile Motion

Projectile - Something thrown or fired. (balls, darts, missiles, bullets, cannon balls)

Trajectory - the path of a projectile. An object projected horizontally in the earths gravitational field will follow a parabolic trajectory.

If we ignore air resistance a projectile has

Horizontally Vertically

- No force Gravitational force downwards only
- No acceleration Acceleration downwards
- constant velocity Increasing velocity downwards

## P5d - Momentum

Every action has an equal and opposite reaction.

Momentum = Mass X Velocity

The greater the mass or velocity of a body, the greater its momentum

Force = change in momentum / time

If the change in momentum happens over a short time, the force of the object will be large and vice versa.

The momentum before a collision = The momentum after the collision. This is conservation of momentum. This also applies to explosions, the total momentum before the explosion is 0. After the explosion the pieces all go in different directions, but the total momentum of all the pieces added together is still 0.

The total momentum of a bullet and a gun is 0. When the bullet is fired it is given momentum in one direction; the gun moves in the opposite direction keeping the total momentum at 0. The movement of the gun is known as the recoil.

## P5d - Momentum Continued...

Safety - If a car stops suddenly the passengers still have momentum. They suffer less injury if momentum is changed slowly.

Seat belts - Stretch slightly, increasing the stopping time, so reducing the force on the passenger

Air bags - Inflate, slowing down the passenger

Crumple zones - fold in collisions, increasing the time taken to completely stop.

## P5e - Satellite Communications

Electromagnetic waves - Waves used to communicate through the atmosphere, i.e. microwaves and radio waves. This is because they are at the lower frequency end of the spectrum.

- Megahertz (MHz) - one million hertz
- Gigahertz (GHz) - one thousand million hertz

Microwaves are

- Used to transmit information to orbiting satellites and retransmit back to earth at a different frequency
- used because their higher frequencies pass through the ionosphere without reflection
- Only diffracted by a small amount because of their short wavelength

Radio waves

- Their use depends on their wavelength and frequency
- Reflected by the ionosphere if their frequency is less then 30MHz (wavelength 10m)

## P5f - Nature of Waves

Diffraction - The spreading of a wave as it moves through a gap. All waves can be diffracted.Maximum diffraction occurs when the size of the wavelength matches or is equal to the size of the gap/object. Diffraction makes radio waves ideal for broadcasting as longer wavelength radio waves can diffract around hills and the horizon, giving them a longer range. Shorter wavelengths don't diffract as much so cant be detected over the horizon or around a hill giving them a shorter range.

Amplitude modulation (AM) - The process of adding a sound wave to a radio wave. he sound wave is changed into an electrical signal by a microphone. This signal is used to alter the amplitude of the radio wave (carrier wave). The opposite process takes place at a radio receiver.

- Interference - When two waves overlap.This gives an interference pattern
- Constructive interference - if the crests of the waves arrive at a point together, they reinforce to make the wave larger
- Destructive interference - When the trough of one wave arrives at a point with the crest of another wave then they are out of phase. If they have the same amplitude then they cancel each other out.

## P5f - Nature of Waves Continued...

Path difference - the difference in distance travelled by two waves from the same source to reach a particular point. If the path difference is an even number of half wavelengths, constructive interference occurs. If it is an odd number then destructive interference occurs.

Plane polarised - When the vibrations of transverse waves take the place in one plane only (at 90 degrees to the direction of travel of the wave)

## P5g - Refraction of Waves

Refraction - The effect on the wave direction (unless travelling along the normal) when light waves cross the boudary from one medium to another. The wave speed is also altered (wavelength, not frequency).

When a light ray moves from a less dense to a more dense medium the wave speed decreases and the light ray is refracted towards the normal. The opposite happens with more dense changing to less dense.

Refractive index = speed of light in a vacuum / speed of light in the medium

= Sin*i* / Sin*r* where *i* = angle of inccience and *r* = angle of refraction

Dispersion - Different colours of light travel at different speeds. If you shine a light through a triangular glass prism a spectrum is produced. Colours at the blue end of the spectrum refract the most (**B**lue **B**ends **B**est).

Critical angle - when the angle of incidence increases so the angle of refraction increases until the ray is refracted along the boundary between the surfaces.

Critical angle between 2 mediums = Sin*c* = *n*r / *n*i Where *c* = critical angle, *n*r = refractive index of refracting medium and *n*i = refractive index of incident medium

## P5h - Optics

Convex lenses

Focal point - Parallel rays of light which pass through convex lenses refract so they all meet at a point. this is the focal point.

Focal length - The distance from the centre of the lens to the focal point

Real image - Can be formed on a screen as the light rays pass through it. It is inverted (upside down)

Virtual image - Cannot be formed on a screen as the light rays only appear to pass through it. It is upright.

Magnification = image height / Object height

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