D.1- Gravitational Fields

• What can large bodies with volume be treated as when dealing with gravitational forces over great distances?

  Point masses
• What is the 'centre of mass'?

  It is a single infinitesimal point where all mass acts.
• What does Newton's Law of Gravitation state?

  The force of attraction between two masses is proportional to each mass and inversely proportional to the distance between their centres squared.
• What is the value of the gravitational constant G?

  G = 6.67 x 10^-11
• What is the gravitational field strength at a certain point?

  It is the gravitational force per unit mass experienced by a small point mass at that point.
• What do gravitational field lines represent?

  They show the direction a test mass would experience a force when placed at that point.
• What are Kepler's Laws of planetary motion?

  1. Planets move on ellipses with the sun at one focus.
  2. The line joining a planet and the sun sweeps equal areas in equal times.
  3. The period of revolution of a planet is proportional to the 3/2 power of the semi-major axis.
• What are the characteristics of geostationary satellites?

  • Orbits the Earth so that it is stationary with respect to a point on Earth's surface
  • Has an orbital period of 24 hours.
• What is gravitational potential energy at a point defined as?

  It is the work done to move a mass from infinity to that point in the field.
• Why is gravitational potential energy always negative?

  Because it is defined as zero at infinity.
• What is escape speed?

  It is the minimum speed an object needs to escape a planet's gravitational pull.
• What happens to an object's energy at any given point in orbit?

  It has half the amount of energy needed to reach infinity.
• What is the relationship between gravitational potential energy and total energy at any given point?

  Ep will always be double ET.
• What are the effects of atmospheric drag on satellites?

  • Satellites experience a drag force.
  • Kinetic energy decreases due to friction.
  • Lower orbit and radius increases speed.
  • Denser atmosphere closer to Earth increases drag.
  • Continues to fall and burns up
• What is gravitational potential?

  It is the work done required per unit mass in moving a test mass from infinity to a point in the field.
• What are equipotential surfaces?

  • Points with the same gravitational potential.
  • Perpendicular to field lines.
  • Cannot cross or meet.
• What is the potential gradient?

  It is the gradient of a potential vs position graph, equal to gravitational field strength.