5.4 gravitational fields

• what causes gravitational fields?

  gravitational fields are due to objects having mass
• what type of force will objects in a gravitational field experience?

  attractive force
• what is gravitational field strength?

  gravitational force exerted per unit mass on a small object placed within the field
• what is the equation for gravitational field strength for a unit mass?

  g = F/m
• what are the units for gravitational field strength?

  N kg ^-1
• what is another way to express Nkg^-1?

  ms^-2
• what is the acceleration due to free fall of an object in a gravitational field?

  gravitational field strength
• what is the direction of gravitational field lines?

  towards the centre of mass
• what are the characteristics of a uniform gravitational field?

  field lines are equally spaced and perpendicular to the surface
• what is newtons law of gravitation?

  newtons law of gravitation states the force between two point masses is :

  • directly proportional to the product of both mass
  • inversely proportional to the square of their separation
• what are the components of this equation?

  F = force between two point masses

  G = gravitational constant

  Mm = product of the two point masses

  r = distance between two point masses

  
• what does the negative sign in the equation for newtons law of gravitation represent?

  that gravitational force is attractive
• what is the gravitational constant?

  G
• what happens to the force when the distance between two point masses is halved?

  force increases by a factor of four
• what is the equation for gravitational field strength between two objects?

  g = -GM / r^-2
• what are the components of this equation?

  g = gravitational field strength

  G = gravitational constant

  M = mass of the larger object

  r = distance between two objects

  
• what does the minus is this equations show?

  gravitational field strength is in the oposite direction to displacement

  
• what are the characteristics of gravitational field strength in a radial field?

  • directly proportional to the mass of the object creating the gravitational field
  • inversely proportional to the square of the distance from the centre of mass of the object
• why doesn't gravitational field strength change from the surface of the earth to the top of a mountain?

  distance is negligible compared to the distance from the surface of the earth to its centre
• What is Kepler's 1st law?

  the orbit is an ellipse with the sun at one of the two foci
• what is Kepler's second law?

  a line segment from the sun to a planet sweeps out equal areas in equal intervals of time
• what is Kepler's third law?

  the square of the orbital period T of a planet is directly proportional to the cube of its average distance r from the sun
• what is an astronomical unit?

  average distance from the sun to the earth
• what is the value of 1 AU?

  1.50 x 10^ 11 m
• how do we get keplers third law?

  gravitational field strength = centripetal force
• what are uses of satellites?

  • communications
  • scientific research
  • weather and climate
  • global positioning
• what are the characteristics of a geostationary satellite?

  • it is in orbit above the earths equator
  • it rotates in the same direction as the earth
  • it has an orbital period of 24 h
• what are some uses of geostationary satellites?

  • driverless vehicles - allow vehicles to function without a driver
  • mining - can find out accurate positions
• what is gravitational potential?

  the work done per unit mass to move an object to a point from infinity
• what is the units of gravitational potential?

  J kg^-1
• what type of quantity is gravitational potential?

  scalar
• why is gravitational potential negative?

  energy is required for a mass to move to infinity
• what is the value of gravitational potential at infinity?

  zero
• what are the factors that effect gravitational potential

  • distance r from the mass producing the gravitational field
  • the mass M
• what is gravitational potential energy?

  work done on a mass to move it from infinity to a point
• what is the relationship between gravitational potential and gravitational potential energy?

  E = m x Vg
• what is the symbol for gravitational potential?

  Vg
• what does area under a Fx graph for a point of spherical mass represent?

  work done
• what is escape velocity?

  velocity required for a mass to escape a gravitational field
• how do you work out escape velocity? 


  kinetic energy = gravitational potential energy