Astrophysics

• How does the inverse square law apply to gravitation?

  The acceleration due to gravity quarters if the distance from the centre of the earth doubles
• Equipotential lines are always at right angles to field lines
• Define gravitational field at a point

  The force per unit mass at the point
• Define conservative field

  When the work done in moving an object through the field is independent of the path taken
• Gravity is a conservative field
• Define gravitational potential at a point in space

  The work done in moving a unit mass from infinity to that point in space
• At which point do both gravitational potential and gravitational potential energy have values of 0?

  At infinity
• define escape velocity

  the minimum velocity required to allow a mass to escape a gravitational field (and have 0 gravitational potential energy)
• Values for gravitational potential are always negative
• Define a black hole?

  a body where the escape velocity exceeds the speed of light
• What frames of reference does special relativity apply?

  inertial (non-accelerating)
• what frames of reference does general relativity apply to?

  non-inertial (accelerating)
• define the equivalence principle?

  no observer can tell by experiment if they are stationary in a gravitational field or in an accelerating frame of reference
• describe the effect of placing a clock at high altitude in a gravitational field?

  clock runs faster
• on a space time diagram, what is the name given to lines representing an objects motion?

  world lines
• define geodesics?

  the shortest distance between two events in space time
• define schwartzchild radius?

  the radius of a spherical mass from which light just escapes (also known as the event horizon)
• define gravitational lensing?

  Deflection of light by gravity
• describe the effect a mass has on space time

  curves and distorts
• what is a light cone in a space time diagram

  an area where an event can affect and be affected by
• Is light at the event horizon redshifted or blue shifted?

  Redshifted
• what are proofs of general relativity?

  1. precession of mercurys orbit
  2. GPS
  3. gravitational lensing
  4. gravitational redshift/blueshift
• what 2 quantities does the luminosity of a star depend on?

  radius and surface temperature
• Units for luminosity?

  Watts (W)
• units for apparent brightness

  Wm^-2
• Define apparent brightness?

  The value of energy per second from the star landing on 1 square metre at the surface of the earth
• How long ago did the solar system form

  4.5 billion years ago
• complete the labels

  A dwarfs

  B main sequence

  C giants

  D supergiants

  
• What process occurs on main sequence stars?

  hydrogen fusion
• What is stage 1 of the proton-proton chain?

  • 2 protons are fused to form deuterium
  • deuterium undergoes beta decay
  • One proton becomes a neutron
  • Positron and neutrino are released at the same time
• What is stage 2 of the proton-proton chain?

  • 2 deuterium and 2 protons fuse
  • He-3 is formed
  • Gamma wave released
• Stage 3 of the proton-proton chain?

  • 2 He-3 fuse
  • Form helium and 2 protons
• State what is meant by the gravitational field strength (g)

  The force per unit mass exerted by gravity.
• State what is meant by the gravitational potential of an object.

  The work done in moving 1 kg from an infinite distance to that point in the field.
• State the units for gravitational potential

  J / kg
• State what is meant by the gravitational potential energy of an object.

  The work done in moving an object from an infinite distance to that point in the field.
• State the unit of potential energy.


  Joules (J)
• Why does potential energy have a negative value?

  It describes the energy we must add to move the object to an infinite distance (therefore zero potential energy)
• State what is meant by the escape velocity of a mass.

  The velocity required to escape the mass' gravitational field and escape to an infinite distance
• What is a black hole?

  An object with an escape velocity that equals/exceeds the speed of light