7.2 Gravitational Fields Flashcards
what is Newton’s law?
Gravity acts on any objects which have mass and is always attractive.
what does Newton’s law of gravitation show?
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.
What are the two types of gravitational field?
a uniform field or radial field
What are the arrows used for in both diagrams of gravitational fields?
The arrows on the field lines show the direction that a force acts on a mass in a force field
What is 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.
What is a radial field?
the force exerted depends on the position of the
object in the field - as an object moves further away from the centre, the
magnitude of force would decrease because the distance between field lines decreases
Gravitational field strength (g)
the force per unit mass exerted by a gravitational field on an
object
What equation do you use for only radial fields
g = GM/r^2
Define gravitational potential at a point
is the work done per unit mass when moving an object
from infinity to that point.
When is Gravitational potential zero?
At infinity , 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.
Gravitational potential (V) equation
V = -GM/r
where r is the distance between the centres of the objects.
What is gravitational potential difference( V Δ)?
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.
Work done equation
Work done = mΔV
Where m is the mass of the object moved
What are equipotential surfaces?
- 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.
What can you calculate from the Gravitational Potential graph?
Gravitational field strength (g)
What is the gravitational field strength graph equation
g = −ΔV/Δr
What happens 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.
What is a force field?
A region where an object will experience a non-contact force.
Describe the gravitational field of the Earth.
- It is radial, so the field lines meet at the centre of the Earth like a spiderweb
- Close to the surface, the field can be considered almost uniform since the field lines are almost parallel and equally spaced
What does Earths radial field look like?
What is the equation of the time period of earths orbit?
T = 2πr/v
What is escape velocity?
- The velocity at which an object’s kinetic energy is equal to minus its gravitational potential energy
- It is the minimum velocity at which an object must travel in order to escape a gravitational field
Why is potential negative?
Have to do work against the field to move an object out of it.
What is an object’s total energy when it travels at escape velocity?
- Zero
* Because the kinetic energy and GPE sum to 0 (since GPE is always negative)
What is the equation for escape velocity?
v = √(2GM/r)
Derive the equation for escape velocity.
- KE = 1/2mv²
- GPE = -GMm/r
- find v
What force keeps an object undergoing circular motion in orbit?
Centripetal force
Why is a satellite’s energy constant in circular orbit?
- Speed and distance above the Earth do not change
- So the kinetic energy and potential energy are constant
- So the total energy is always constant
Why is a satellite’s energy constant in elliptical orbit?
- The satellite speeds up as it’s orbital radius decreases and slows down as orbital radius increases
- So kinetic energy increases as potential energy decreases (and vice versa)
- So the total energy remains constant
What is a synchronous orbit?
Where the orbital period is the same as the rotational period of the orbited object.
What is the time period of orbit of a geostationary satellite?
1 day
What are geostationary satellites used for?
Sending TV and telephone signals.
Compare the advantages of low orbit satellites and geostationary satellites.
Low orbit
* Cheaper to launch
* Require less powerful transmitters since they are close to Earth
Geostationary
* Do not require multiple satellites to achieve constant reception in one area
