P5 - Space for reflection

Scalar quantity: quantity that has only a number e.g. speed, mass, time, temperature, length

Vectory quanitity: quantity with both direction and magnitude e.g. velocity, force, acceleration

Equations of motion:

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• write down three things you already know
• write down the other things you want to find out
• choose equation that involves all things you've written down
• do equation after substituting numbers

The path of a projectile is always a parabola:

• if angle is less than 45 degrees, projectile doen't travel very far and if angle is greater than 45 degrees than the projectile will take longer to hit the ground and won't travel as far

Horizontal and vertical motion:

• an object projected horizontally accelerates vertically due to gravity but has no horizontal acceleration

Forces occur when two objects interact:

• if you push something, it will push you back
• newton's third law of motion: for every action, there is an equal and opposite reaction
• Recoils:
  • when gun fired, bullet exerts force equal and opposite to force exerted by gun so bullet travels out of barrel and gun recoils in opposite direction and bullet travels faster because it is lighter
• Rockets:
  • in engine, particles of hot gas collide with walls exeerting force on walls and walls exert equal and opposite force on them
  • force on wall pushes gas particles out of exhause and force from gas on wall pushes rocket forwards
  • force pushing rocket upwards must be larger than force of gravity amd air resistance otherwise it wont take off

Momentum = mass x velocity

• greater mass and greater velocity = more momentum

Conservation of momentum:

• if two objects collide and coalesce, the total momentum of both objects before the collision = momentum of the combined objects after the collision

  • (m₁ x u₁) + (m₂ x u₂) = (m₁ + m₂) x v

  • m1 = mass of first object
  • u1 = velocity of first object
  • m2 = mass of second object
  • u2 = velocity of second object
  • v = velocity of combined object = (added velocties if in same direction and subtracted velocities if in differnet direction)

Kinetic theory:

• gases consist of small, constantly moving particles moving in random directions

Decrease in volume gives an increase in pressure:

• particles are more crowded together and so hit container walls more often = an increase in pressure

Increase in temperature means an increase in pressure:

• particles have more kinetic energy so hit the container walls harder and more often = more pressure

Colliding particles change their momentum:

• gas particles move and have mass so have momentum = M = m x v

Centripetal orbits:

• orbit = a balacne between the forward motion of an object and centripetal force pulling it inwards

Gravity decreases when you get further away:

• the closer you are to a planet, the stronger the gravitational force of attraction
• force decreases very quickly with increasing distance - F µ 1/d²
• double distance from planet = gravitational force decreases by a factor of four
• treble distance from planet = gravitational force decreases by a factor of nine

Comets:

• have elliptical orbits
• as the comet gets closer to the Sun, its speed increases and as it goes farther away from the Sun, its speed decreases.

Geostationary satellites:

• high orbit over the equator, orbit once every 24 hours and stay at the same point above Earth
• ideal for communication satellites such as telephones, TVs and radios

Low polar orbits:

• satellite passes over both poles while Earth rotates underneath
• muich closer to Earth than geostationary orbit so pull of gravity and stronger therfore travel much faster and orbit very quickly which allows whole surface of Earth to br monitored quickly

Microwaves:

• used for communication - signal transmitted into space, revieved by reciever and re-transmitted in different direction to recieve in dish on Earth
• frequency = over 3000MHz, pass through atmosphere easily
• satellites use lower frequencis than satellites in higher orbits, digital signals are better to use

Different frequency waves travel by different routes:

• Below 30MHz - radio waves reflected off ionosphere - allows wave to travel longer distances and deal with curvature of Earth
• Between 30MHz and 30GHz - radio waves and microwaves pass straight through atmosphere so transmissions must be in line of sight
• Above 30GHz - rain and dust in atmosphere absorb and scatter microwaves which reduces strength of signal so highest frequency that can be used for satellite transmissions os 30GHz

Long wavelength radio waves diffract:

• longer wavelength = more diffraction
• maximum diffraction = when the size of the gap is the same as the wavelength of the wave
• long wavelength radio waves have a really large wavelength as well as a long range so are good for broadcasting

When waves meet, they cause interference:

• constructive interference = waves disturb in same direction and reinforce each other
• destructive interference = waves disturb in opposite directions and cancel each other out

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To get a stable interference pattern you need to use coherent wave sources where:

• waves are in same frequency
• waves are in phase - troughs and crests of waves line up
• waves have same amplitude
• light = monochromatic light

Diffraction of light = patterns of light and dark:

• when a wavefront passes through a gap light from each point along the gap diffracts
• gap must be about same size as wavelength of light
• diffracted light from each of these points interferes with light diffracted from all other points
• pattern has bright central fringe and alternating dark and bright fringeson either side

Light is a wave:

• 17th century - 1) Isaac newton - particle theory 2)Christiaan Huygens - wave theory
• particle theory explains reflection and refraction and wave theory explains diffraction and interference - thomas young's double slit experiment shows that light could both diffract anf interfere
  • light source shone through two slits = pattern of light and dark fringes showing constructive and destructive interference = wave behaves liek a wave

Plane polarisation: light is passed through polarising filter so filter transmits vibrations in one direction

Refraction: a wave changes direction as it eneters a different medium as a result of its changing speed

• when wave speed decreases, wave bends towards the normal
• when wave speed increases, wave bends away from the normal

Refractive index = ratio of speed of light in a vacuum to speed of  light in a medium (3/2 for glass)

• higher refractive index = more light bends when it enters/leaves the medium

Refractive index = 

.

Dispersion:

•  when light travels from air to glass:
  • red light slows down the least so it's refracted the least and has the lowest refractive index
  • blue light has a higher refractive index so is refracted more
  • a prism causes different colours of white light to emerge at differnt angles
  • this produces a spectrum showing all colours of the rainbow = dispersion

Three different conditions:

• angle of incidence LESS than critical angle:
  • most light passes through into air but a little bit is internally reflected
• angle of incidence = critical angle:
  • emergin ray travels alomng surface of medium, some light internally reflected
• angle of incidence GREATER than critical angle:
  • all light is internally reflected

Real image: light from an object comes together to form an image on a screen, can be projected on screens

Virtual image: rays are diverging so light from the object seems to be coming from a completely different place, can't be projected

Describing an image: how big it is compared to the object, whether it's upright or inverted, whether it's real or virtual and where

Convex lenses focus light:

• = converging lens - causes light to converge to a focus
• if rays entering lens are parallel to each other and to principal axis, it focuses them at the focal point
• distance between centre of lens and focal point = focal length 

"Draw a ray diagram to show the image from a convex lens"

• draw in focal length
• pick point on top of object and draw ray going from object to lens parallel to principal axis 
• draw another ray from top of object through the centre of lens
• draw refracted ray passing through focal point
• ray passing through centre of lens doesn't bend so draw line staright through middle of lens
• mark where rays meet = top of image
• repeat for point on bottom of object

Distance from lens affects image:

• distance at 2F produces real upside down image that is the same size at the object and at 2F
• between F and 2F it will make real, upside image bigger than object and beyond 2F
• object nearer than F will make virtual image right way up, bigger than the object on the same side of lens

Convex lens are used in magnifying glasses to create magnified images:

• object being magnified must be closer to lens than focal length
• image produced = virtual image

Cameras:

• when taking photographs, light from object travels to camera and refracted by lens, froming image on light sensor
• image on sensor is real image and is smaller than object because object is further away than focal length of lens, image is also inverted

Projectors:

• when projecting images, object need to be placed upside down and very close to lens
• light from object refracted by lens and produces real, inverted and magnified image on screen

Magnification =