The moon orbits around earth and earth and other planets orbit around the sun because of the gravitational force between masses. A cetripetal force acts towards the centre of a circle;it keeps an object moving in a circle. Gravity provides the centripetal force that keeps satellites in orbit.
Inverse Sqaure Law:
If the distance doubles, the force drops to 1/4. (Gravitational force gets weaker as the objects move further away).
Variation of the speed of a comet:
- influence of a highly eliptical orbit
- when the periodic comet is near to the sun it travels very fast to escape the gravitational force. When it gets further away, it goes slowly because the Sun's gravity pulls it back.
Planets close to the sun have short periods - further away have long periods.
Geostationary Artificial Satellites:
- orbits the Earth in 24 hours around the equator
- remains in a fixed position above the Earth's surface
- orbits above the Earth's equator
Artificial Satellites are continually accelerating towards the earth due to gravity, but their tangential motion keeps them moving in an approximately circular orbit.
Satellite applications require different orbits:
The height and the period of a satellite determines what it is used for:
Low polar orbit = imaging earth's surface, weather forcasting and military uses
Geostationary orbits = communications and weather forcasting
Scalar - direction is not relevent - mass, time
Vector - direction is important - force, velocity, acceleration
- Parallel vectors add.
- Vectors in opposite vectors subtract
- two forces acting at right angles - use pythagoras' theorem
ALL SUVAT EQUATIONS - IN THE QUESTION PAPER
The trajectory of an object projected in the Earth's gravitational field is parabolic.
The horizontal and vertical velocities of a projectile are vectors.
The resultant velocity of a projectile is the vector sum of the horizontal and vertical velocities.
For a projectile in the Earth's gravitational field (ignoring air resistance):
- there is no acceleration in the horizontal direction - a constant horizontal velocity
- the acceleration due to Gravity acts in the vertical direction (steadily increasinf vertical velocity)
Other than air resistance, the only force acting on a ball in flight is gravity.
Projectiles have a downward acceleration and this only affects the vertical velocity.
Projectile Motion 2
For an object projected horizontally:
- the horizontal velocity is unaffected by gravity
- therefore the horizontal velocity is constant
- gravity causes the vertical velocity to change
Action and Reaction
When an object collides with another object or two bodies interact, the two objects exert an equal and opposite force on each other (Newton's 3rd Law of Motion).
When you stand on the ground:
- you exert a force on the ground (our weight)
- the ground exerts an equal and opposite force on you (contact force)
Momentum is always conserved:
Recoil - total momentum of gun and bullet is 0. The bullet moves faster but has a smaller mass, so their momenta are equal but in opposite directions - cancelling eachother out.
Explosions - before total momentum is 0. After an explosion each fragment flies off in different directions.Momentum of one fragment will cancel out another fragment travelling in different direction - cancelling eachother out.
Action and Reaction 2
Rocket propulsion - when a rockets engines fire into space, the rocket speeds up, but the total momentum of the system is conserved. This is because the forward momentum of the rocket is cancelled out bu the backward momentum of the gas it fires out.
m1u1 + m2u2 = (m1 + m2) v
A change in volume or temperature produces a change in the pressure:
- squashed into smaller volume - each particle will collide with the wall more frequently, the pressure inside the container increases.
- temperature increased - increased kinetic energy, each particle will collide with the wall more frequently and with more force, increasing the pressure.
Action and Reaction 3
- the change in momentum of the particles stiking the walls creating a force.
- the greater the number of collisions with the wall, the higher the pressure.
Rocket propulsion involves fast moving particles colliding with the rocket walls creating a force.
For large scale rockets used to lift satellites into the Earth's orbit, sufficient force is created to lift the rocket:
- a large number of particles of exhaust gas are needed
- the particles must be moving at high speeds.
Information can be transmitted using microwaves to orbiting artificial satellites and then retransmitted to Earth or to other satellites.
The satellite transmitting and receiving dishes need very careful alignment:
- the size of the satellite communication dish is many times the microwave wavelength
- this produces little diffraction hence a narrow beam that does not spread out
- this means that the receiving dish and satellite dish need exact alignment
Satellite Communication 2
Electromagnetic waves with different frequencies behave in the atmosphere differently:
- below 30MHz are reflected by the atmosphere
- above 30GHz, rain, dust and other atmospheric effects reduce the strength of the signal due to absorption and scattering
- between 30MHz and 30GHz can pass through the earths atmosphere.
Wave patterns produced by a plane wave passing through different sized gaps:
- gap larger than wavelength - slight diffraction
- gap same size as wavelength - maximum diffraction
Very long wave radio waves have a very long range because it is relatively easy for radio waves to diffract around obstacles and the horizon.
Reinforcement - where waves are in phase and they add together. This is also known as constructive interference. This produces a wave with a larger amplitude so you may see bright fringes. EVEN number of half wavelengths.
Cancellation - where waves are not in phases and they subtract from each other. This is also known as destructive interference. Dark fringes when the amplitude is zero. ODD number of half wavelengths.
Coherent wave sources are needed to produce a stable interference pattern.
For light, the coherent sources are monochromatic light.
Properties of coherent wave sources:
- same frequency
- in phase
- same amplitude
In such a double-slit experiment, the two sets of diffracted waves interfere with one another and form an interference pattern. This is seen on a screen as a pattern of bright and dark bands, also known as fringes:
bright bands are seen where constructive interference has happened
- dark bands are seen where destructive interference has happened
Interference patterns are also set up when light passes through a single slit. This is because waves from different parts of the slit interfere with one another. The pattern of bands or fringes is slightly different from the one seen in a double-slit experiment:
the central bright fringe is wider than the other bright fringes
the central fringe is brighter than the other fringes
These diffraction experiments prove the nature of light waves - they travel in straight lines.
Plane polarised light - light that is only oscillating in one plane (direction)
All electromagnetic waves are transverse and so can be plane polarised.
If a material is a horizontal polariser:
- only horizontal oscillations can get through
- other oscillations are absorbed
Used in some sunglasses to reduce the glare from sunlight. The light that gets through is plane polarised and, therefore, less bright.
When light is incident on water, the reflected light is partly plane polarised.
Light - Wave or Particle?
The particle theory is not universally accepted because scientists believed that light is a series of waves. This theory was supported by experiments demonstrating diffraction of waves, interference and dispersion (splitting white light into a spectrum)
The particle theory could explain reflection but not interference patterns.
Refraction occurs at a boundry between two media:
- when a wave speed decreases the wave bends towards the normal
- when a wave speed increases the wave bends away from the normal
The refractive index is a measure of the amount of bending after a boundary.
refractive index = speed of light in vacuum/speed of light in medium
The amount of bending increases with greater change of wave speeds in different media but the same speeds in a vacuum.
Dispersion of light into the spectral colours:
- light has a different speed in glass
- blue light has a greater refractive index than red light so is slowed down the most. So blue light is deviated (changed direction) more than red light.
Light incident on a glass/air surface when the angle of incidence is:
- less than the critical angle - most of the light refracted into the air
- equal to the critical angle - maximum refraction emerging 90 degrees to the normal
- more than the critical angle - no light refracted, all light is reflected back into the medium - total internal reflection.
- optical fibres
- reflectors and cat eyes on the road and road signs.
Different media have different critical angles.
Conditions for TIR to occur:
- angle of incidence is greater than the critical angle
- light from a medium with a high refractive index into a medium with a low refractive index
The refractive index of a medium relates to its critical angle :
- the higher the medium's refractive index, the lower its critical angle.
- sin c = Nr/Ni
- Nr = refractive index of air
- Ni = refractive index of the medium
Effects of a convex lens on:
- diverging rays - converges the rays (brings them together and brought to a focus.
- a paralell beam of light - all focused to a single point
Parts of a convex lens/diagram:
- principal axis - through the optical centre of the lens
- focal length - The distance from the centre of the lens to the focal point
- focal point - Light rays passing through the lens parallel to the principal axis are focused to a single point
- optical centre of the lens
Convex lens produce real images.
A convex lens refracts light. There are three situations you need to know about:
A ray travelling parallel to the principal axis before it reaches the lens is then refracted through F on the other side of the lens.
A ray travelling through F before it reaches the lens becomes parallel to the principal axis when it reaches the other side.
A ray which passes directly through the centre of the lens travels straight through to the other side.
Uses of a convex lens:
- in a camera
- in a projector
- as a magnifying glass
For the image to be in focus, the lens has to be moved so that they images forms on the screen where the rays form a particular point on the object meet at a point on the screen.
magnification= image size/object size
Real images are upside down (inverted) and can be projected onto a screen.
Virtual images are the right way up, but cannot be projected onto a screen.