A2 Physics Gravitational fields

A2 Physics Gravitational fields

1. Representing a gravitational field

   • Gravitational field lines
     • The arrows on the field lines show us the direction of the gravitational force on a mass placed in the field
     • The spacing of the field lines indicate the strength of the gravitational field
   • Earth's gravitational field
     • Gravitational force gets weaker as object travels further away from Earth as shown by separation between the field lines
     • The gravitational fields act as if the entire mass of the Earth is concentrated at its center
     • Closer to its surface, the gravitational force is directed directly downwards and evenly spaced so the strength of the field is the same at all points
     • The Earth's gravitational field is radial since the field lines diverge radially from the center of the Earth
     • On a smaller scale closer to the surface, the gravitational field is uniform since the field lines are equally spaced
   • Newton's law of gravitation
     • Any two point masses attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of their separation
     • The bigger the masses and the smaller the separation - the bigger the gravitational force

2. Gravitational field strength g

   • The gravitational field strength at a point is the gravitational force exerted per unit mass on a small object placed at that point
   • Is a vector so direction is also needed
   • N/kg or m/s^2

3. Orbiting under gravity

   • Satellites are able to orbit a mass because of the gravitational force acting as the centripetal force
   • Satellite follows a circular path because the gravitational force is at right angles to its velocity
   • The centripetal force must equal the gravitational force giving the equation

4. Orbital period

   • Since velocity is equal to the circumference/orbital period of a satellite we can substitute to give

5. Orbiting the Earth

   • Earth has many satellites: natural and artificial
     • As it follows the velocity equation, the closer the satellite is to the Earth's surface, the faster it will get
   • Observing the Earth
     • We have many satellites for different purposes and can follow two orbits: circular or elliptical
   • Geostationary orbits
     • This is a special type of orbit where the satellite is positioned so that it and the Earth rotate at the same rate and remains above a fixed point on its equator
     • Eventually they will drift out of the correct orbit so rockets are needed to be put back in place, this fuel can run out so it has to be replaced every 10 or so years
     • We can determine the distance of a satellite in geostationary orbit using Kepler's third law