Gravitational Fields Flashcards
(20 cards)
Force field
A force field is an area in which an object experiences a non-contact force. Force fields can be represented as vectors, which describe the direction of the force that would be exerted on the object, from this knowledge you can deduce the direction of the field.
They can also be represented as diagrams containing field lines, the distance between field lines represents the strength of the force exerted by the field in that region.
Gravitational fields
formed during the interaction of masses
Electric fields
formed during the interaction of charges
What are gravitational and electric field similarities and differences?
How does gravity act?
Gravity acts on any objects which have mass and is always attractive.
Newtons law of gravitation?
Newton’s law of gravitation shows that the magnitude of the gravitational force between two masses is directly proportional to the product of the masses, and is inversely proportional to the square of the distance between them, (where the distance is measured between the two centres of the masses).
Where G is the gravitational constant, m1 /m2 are masses and r is the distance between the centre of the masses.
A uniform field
The arrows on the field lines show the direction that a force acts on a mass. A uniform field exerts the same gravitational force on a mass everywhere in the field, as shown by the parallel and equally spaced field lines.
radial field
In a radial field the force exerted depends on the position of the object in the field, e.g in the diagram above, as an object moves further away from the centre, the magnitude of the force would decrease because the distance between the field lines increases. The Earth’s gravitational field is radial, however very close to the surface it is almost completely uniform
Gravitational field strength (g
Gravitational field strength (g) is the force per unit mass exerted by a gravitational field on an object. This value is constant in a uniform field, but varies in a radial field. There are two formulas you can use to calculate this; the first is general, while the second is used only for radial fields:
Gravitational potential (V)
Gravitational potential (V) at a point is the work done per unit mass against gravitational force to move an object from infinity to a given point. Gravitational potential at infinity is zero, and as an object moves from infinity to a point, energy is released as the gravitational potential energy is reduced, therefore gravitational potential is always negative.
The gravitational potential difference VΔ
The gravitational potential difference VΔ is the energy needed to move a unit mass between two points and therefore can be used to find the work done when moving an object in a gravitational field.
Equipotential surfaces
are surfaces which are created through joining points of equal potential together, therefore the potential on an equipotential surface is constant everywhere. As these points all have equal potential, the gravitational potential difference is zero when moving along the surface, so no work is done when moving along an equipotential surface. The red lines on the diagram to the right represent equipotential surfaces.
What does the equation for gravitational potential tell us?
gravitational potential (V) is inversely proportional to the distance between the centres of the two objects (r). This can be represented on a graph of potential (V) against distance r:
If you plot a graph of gravitational field strength (g) against distance (r), you can find…
the gravitational potential difference by finding the area under the curve.
Kepler’s third law
Kepler’s third law is that the square of the orbital period (T) is directly proportional to the cube of the radius (r)
Total energy of a satellite
The total energy of an orbiting satellite is made up of its kinetic and potential energy, and is constant. For example, if the height of a satellite is decreased, its gravitational potential energy will decrease, however it will travel at a higher speed meaning kinetic energy increases, therefore total energy is always kept constant.
The escape velocity
The escape velocity of an object is the minimum velocity it must travel at, in order to escape the gravitational field at the surface of a mass. This is the velocity at which the object’s kinetic energy is equal to the magnitude of its gravitational potential energy.
A synchronous orbit
is one where the orbital period of the satellite is equal to the rotational period of the object that it is orbiting, for example a synchronous satellite orbiting Earth would have an orbital period of 24 hours.
Geostationary satellites
Geostationary satellites follow a specific geosynchronous orbit, meaning their orbital period is 24 hours and they always stay above the same point on the Earth, because they orbit directly above the equator. These types of satellites are very useful for sending TV and telephone signals because it is always above the same point on the Earth so you don’t have to alter the plane of an aerial or transmitter.
Low-orbit satellites
Low-orbit satellites have significantly lower orbits in comparison to geostationary satellites, therefore they travel much faster meaning their orbital periods are much smaller. Because of this, these satellites require less powerful transmitters and can potentially orbit across the entire Earth’s surface, this makes them useful for monitoring the weather, making scientific observations about places which are unreachable and military applications. They can also be used for communications but because they travel so quickly, many satellites must work together to allow constant coverage for a certain region.
