Chapter 18 gravitational fields Flashcards
What are gravitational foibles due to?
Mass
How is the mass of a spherical object modelled?
As a point at the centre of mass
What do all objects with mass create?
A gravitational field
Where do gravitational fields extend to?
Infinity (but gets weaker as distance from centre of mass increases, becoming negligible at long distances )
What will any object with mass experience in a gravitational field?
An attractive force towards the centre of mass of the object creating the field (the object with the larger mass out of the 2 masses)
What is the gravitational field strength equal to on earth?
9.81ms^-2 (free fall)
Is gravitational force always attractive?
Yes
How do we represent a stronger gravitational field with field lines?
The field lines are closer together
What do field lines form around a spherical mass?
A radial field
What properties do field lines have if there is a uniform gravitational field?
They are:
-equidistant ( equal distance between the field lines)
-parallel
Does the gravitational field strength change in a uniform field?
No
What are the gravitational field lines close to the surface of a planet?
Approximately uniform (simply straight lines towards the centre of mass which are parallel and equidistant, as when you get close enough to a planets surface the surface simply becomes flat)
What does Newtons law of gravitation state?
That the force between 2 masses is:
-directly proportional to the of the masses( F proportional to Mm)
[larger M is the mass that attracts the other mass i.e. has a larger mass and m is the mass that experiences the gravitational attraction from M]
-inversely proportional to the square of their separation (F is proportional to 1/r^2)
What does the minus sign represent in the equation for newtons law of gravitation?
It shows that the gravitational force is an attractive force.
How do i find the resultant force of an object, or multiple objects?
Model them as vectors and use vector addition and subtraction while some cases may require you to use a force diagram.
How do you derive the equation for gravitational field strength in a radial field?
Use g=F/m And F =-GMm/r^2 Sub F
Get g=(-GMm/r^2)/m The m’s cancel
Get g=-GM/r^2
What does the negative sign represent in the derived equation for gravitational field strength in a radial field?
It shows that the gravitational field strength at that point is in the opposite direction to the displacement r from the centre of mass( a gravitational field is an attractive field)
What is Keplers first law?
The orbit of a planet is an ellipse with the sun at one of 2 foci points ( foci = the two bits towards the end of an ellipse but still in the middleish)
What is the aphelion?
The furthest point from the sun in an orbit
What is the perihelion?
The closest point to the sun in an orbit
What is meant by the level of eccentricity when looking at somethings orbit?
It is a measure of how elongated a circle is
-low eccentricity= circle
-high eccentricity= elliptical
What is Keplers second law?
A line segment joining a planet and the sun sweeps out equal areas during equal intervals of time.
(As planets move around the sun the speed is not constant, planet closer to sun=move faster)(time interval=same)
What is Keplers third law?
The square of the orbital period is directly proportional to the cube of its average distance form the sun
What is one astronomical unit?
The mean distance between the earth and the sun
How do i work out the orbital speed of a planet?
-Equate the centripetal force of the planet to the gravitational force on the planet and rearrange for v.
-2pi divided by time = velocity (2pi=circumference=distance)
How do you derive the orbital speed for a stable distance r from the centre of mass of the earth?
By equating the centripetal force and the gravitational force
-giving you v=square root (GM/r)
Mass of the satellite is not a factor in this equation
Even if the mass of a satellite varies will all satellites at a given height be travelling at the same speed?(in a given orbit)
Yes it will be the same (the mass is not taken into account)
What do satellites with a polar orbit circle?
The poles
What are polar satellites used for?
-covers all parts of the globe (as earth spins on its axis)
-used for mapping
-reconnaissance (military use)
What 3 points must a satellite follow to be a geostationary satellite?
-be in orbit above the earths equator
-rotate in the same direction as the earths rotation
-have an orbital period of 24 hours
What is the gravitational potential at a point in a gravitational field defined as?
The work done per unit mass to move an object to that point from infinity
What does infinity refer to in terms of gravitational potential?
To the distance so far from the object producing the gravitational field that the gravitational field strength is zero
is gravitational potential a scalar or vector quantity?
It is a scalar quantity
What are all values of gravitational potential?
Negative
Why are all values of gravitational potential negative?
they are all negative because:
-all masses attract each other
-takes energy(external work must be done) to move objects apart
Where is gravitational potential maximum?
At infinity
-value=0Jkg^-1
What is the gravitational potential when r is equal to infinity?
0JKg^-1
What does moving towards a point mass do to the gravitational potential?
Results in a decrease in gravitational potential
What does moving away from a point mass do to the gravitational potential?
Results in an increase in gravitational potential
What is the definition of gravitational potential energy of any object of mass m within a gravitational field?
The work done to move the mass from infinity to a point in a gravitational field
How do you derive the equation for the escape velocity?
Equate the kinetic energy to the gravitational potential energy then rearrange for v
What must be the supplied energy of an object in order to escape a gravitational field?
The energy must be equal to the gain in gravitational potential energy needed to lift it out of the field.
