Our dynamic universe

• acceleration


  the rate of change of velocity
• acceleration of X ms^-2


  The velocity of the body increases/changes by X m s^-1 every second.
• Give a description of the big bang


  The Universe was initially in a hot and very dense state and then rapidly expanded. The universe started from a point/singularity and rapidly expanded.
• Give evidence for the big bang


  Cosmic Microwave Background Radiation, Redshift of galaxies, Olber's Paradox (and darkness of the skies), H-He Problem (relative abundance of hydrogen/helium)
• What is the law of conservation of momentum during a collision


  During any collision, in the absence of external forces momentum is conserved, total energy is conserved. In an elastic collision - Ek is also conserved In an inelastic collision- Ek is transferred into other forms.
• Component weight down a slope


  W sin θ, or mg sin θ
• Give a reason why the existence of this CMB supports the Big Bang Theory.


  is pervasive throughout space. It is the dominant source of radiation in the Universe. It is very uniform (throughout the Universe). It is isotropic (throughout the Universe). It shows the characteristics of blackbody radiation. It has a temperature of approx 3 K (2·74 K) due to cooling on expansion. It corresponds to a redshift of 1000, so the early temperature of this radiation was approx 3000 K. CMBR is thought to be the "afterglow" of the Big Bang, cooled to a faint whisper in the microwave region.
• Dark Energy


  a hypothetical form of energy whose negative pressure counteracts gravity and is assumed to be responsible for the universe expanding at an accelerating rate.
• Dark Matter


  a hypothetical form of matter invisible to electromagnetic radiation, postulated (suggested) to account for gravitational forces observed in the universe.
• Doppler Effect


  The Doppler Effect is the apparent change in frequency of a wave when the source and observer are moving relative to each other.
• Doppler explanation


  There are more wavefronts per second observed due to the forward motion of the vehicle. (NB not wavefronts produced as there are not!)
• Evidence for the big bang

  The cooling of the Universe and cosmic microwave background radiation provide evidence for the Big Bang
• Explain why a star is redshifted.


  • The star is moving away from the Earth

  • The apparent wavelength of the hydrogen spectra from the star has increased

  • The apparent frequency of the hydrogen spectra from the star is less than the actual frequency on Earth

  • The frequency of the light from the star has shifted towards the red end of the spectrum

  • Light from the star is experiencing a Doppler shift.
• Explain why the driving force of a car must be increased with time to maintain a constant acceleration.


  The faster it goes, the greater the air resistance. or frictional forces / friction / drag then F(drive) constant, the unbalanced force would decrease or increasing F(drive) keeps the unbalanced force constant or overall/net force - must have
• Explain, in terms of the forces, why there is a maximum angle of slope that a cart (with engine) can ascend.


  As angle (of slope) increases mgsinθ increases When mgsinθ ≧ engine force - friction, the vehicle cannot move up the slope
• What do the different features of an F-t graph represent?


  The Area under a F-t graph is equal to impulse or change in momentum of the object.
• Hubble Constant Ho


  the ratio of the speed of recession of a galaxy (due to the expansion of the universe) to its distance from the observer.
• inertial reference frame


  two objects that are moving at constant speed with reference to each other
• Law of conservation of momentum


  Total momentum before (a collision) is equal to the total momentum after (a collision) in the absence of external forces
• Length contraction


  The decrease in length (in the direction of motion) of an object moving relative to an observer. N.B. it must be clear that the observer is in a different frame of reference.
• What is the definition of a Newton

  One Newton is equal to the force which causes an acceleration of one metre per second squared when applied to a mass of one kilogram.
• Cosmic Microwave Background (Radiation)


  Radiation of peak wavelength 1·06 mm can be detected on Earth coming from all directions in space.
• Redshift example


  More distant galaxies are moving away at a greater velocity/ have a greater recessional velocity
• Redshift explanation


  Light from objects moving away is shifted to larger wavelength or the rate of change of wavelength/emitted wavelength as the galaxy moves away
• Redshift, z


  the change in wavelength divided by the original wavelength, and given the symbol z. It has no units.
• Reference Frame


  The background frame against which measurements are made. There is no absolute reference frame.
• Resultant force


  A single force that has the same effect as the forces actually acting on an object
• Satellites a curved path


  Constant horizontal speed but are accelerating in the vertical direction under the influence of a gravitational field. Constantly falling towards earth.
• scalar and vector


  A vector is a quantity which has both a magnitude and direction. A scalar is fully described by its magnitude.
• Spectral lines


  A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range. Spectral lines are often used to identify atoms and molecules from their characteristic spectral lines. These "fingerprints" can be compared to the previously collected "fingerprints" of atoms and molecules, and are thus used to identify the atomic and molecular components of stars and planets.
• Terminal Velocity


  A constant velocity of an object when the driving force acting on an object is balanced by the frictional force.
• Time dilation


  Time dilation is a difference in a time interval as measured by a stationary observer and a moving observer.
• twin paradox


  Special Relativity would suggest one twin who has been out in space or travelling at high speed relative to one remaining on Earth should come back younger, but to the travelling twin they could consider themselves stationary as the Earth moved away fast, so to the astronaut twin the twin on Earth should be younger. This paradox is resolved as during the journey in space the travelling twin would have had to return, during which time they would be in a non-inertial reference frame, so they would have experienced the speed/acceleration. Therefore been in a non inertial reference frame and hence special relativity does not apply.
• Blueshift


  Doppler-shifting of a light wave towards the blue end of the spectrum (observed frequency higher than emitted frequency) owing to relative motion of the source towards the observer
• Components of a vector


  two vectors which act at right angles, the vector sum of which is the original vector
• Conservation of energy


  energy cannot be created or destroyed, only converted from one form to another
• Conservation of momentum


  when two or more objects interact, the total momentum is conserved, in the absence of external forces
• Dark energy


  a theoretical form of energy postulated to act in opposition to gravity and to occupy the entire universe, accounting for most of the energy in it and causing its expansion to accelerate
• Dark matter


  thought to be a type of matter which does not interact with electromagnetic radiation so is invisible to astronomers detecting light or any other type of electromagnetic radiation eg radio to gamma
• Displacement


  a specified distance from a fixed point, in a specified direction. Displacement is a vector quantity.