C18 (Gravitational Fields) Flashcards
Gravity
Universal attraction between any two masses
(Relatively weak force, only noticeable when one of masses very large, but also present tiny scale)
What is the gravitational pull excreted on a n object directly proportional to?
The mass
Weight
Force due to gravity
Gravitational force (equation)
Fg=W
Gravitational Field
Vol that gravity will act in
Gravitational field strengths equal to what:
Gravity
W=mg
Gravitational potential energy
Change GPE= mg x change h
‘G’-Universal
Same throughout universe
G, very small, only significant when at least on mass is very large
Why does the gravity equation contain a negative sign?
Indicates that force is attractive
What does the lines in the gravitational field indicate?
Direction of Fgrav would act on test mass
Test mass
Mass, small enough to not have no affect on shape of field with its own gravity
The more lines per area (gravitational field):
The stronger the field
How can you detect very small changes in gravity
Gravimeter
Used map tunnels, caves, change volcanic activity and minerals for mining
For object moving in a circle, what’s it’s resultant force called
The centripetal force
Centripetal force provided by…
Gravitational attraction of M (large body)
Centripetal force is equal to…
Gravitational force
Fc=Fg
Kepler’s first law
The orbit of every planet is an ellipse with the sun at one of the foci.
Perihelion
Closet distance to sun
Aphelion
Furthest distance from sun
Eccentricity
Measure of how elongated the circle is (usually orbits have low eccentricity).
Kepler’s second law
A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time
As planets move on their elliptical orbit their speed….
Isn’t constant
When closer they travel faster
Kepler’s third law
The square of the orbital period (T) of a planet is directly proportional to the cube of its average distance (r) from the Sun
The gradient of a graph with T squared against r cubed is…
4Pi^2 / GM
Satellite
Body that circles round another body in space
Give examples of natural satellites
Moons
Stars
Comets
Asteroids
Planets
Uses of satellites
Communication (satellite phones, not mobile phones, TV, some types of satellite radio)
Military (reconnaissance)
Scientific research (Study universe and monitor Earth, population crops vegetation)
Weather and Climate: (Predicting and monitoring weather across the globe and monitoring long term changes in climate)
Global positioning
Stallites orbiting Earth obey what laws
Kepler’s laws
What provides the centripetal force on each satellite
Gravitational force between it and the Earth
Why don’t the satellites fall towards earth if the only force acting upon them is Fg
Always falling,however travels great distance, that as it falls Earth curves away beneath it, keeping it the same height.
What is required for a stable orbit
Exactly right height and speed
The correct speed for stable orbit at distance r from centre mass of Earth is given by…
V=✓Gm/r
What does every satellite that is placed in a given orbit at a given height have in common
They all travel at the same speed (even if their masses vary)
Once launched they’re normally above atmosphere, where there’s no air resistance meaning…
No air resistance to slow them down, as a result they travel at a constant speed
Polar orbits
Circle poles
Offers complete view Earth, over given period, as Earth rotates beneath, satellites covers all parts globe after number of orbits, useful for mapping and reconnaissance
Low Orbit
Satellites in orbit close to Earth, Kepler’s third law (T squared directly proportional to r cubed)
Only short time taken to orbit Earth
Geostationary satellites
Placed in orbit above equator
As height increases T increases = can choose it’s T by altering height
Geostationary placed in specific orbit, so remains above same point of Earth whilst it rotates
To be in a geostationary orbit satellite must…
Be in orbit above Equator
Rotate same direction as Earth
Have orbital period, 24hrs
The orbital period of geostationary satellites is how long?
24hrs
Gravitational potential (definition):
The potential energy per kg, at any in the field. 0 potential is defined at infinity, so point close to a mass , potential of any object would be -ve.
Gravitational potential energy (definition):
The work done per unit mass in moving object from infinity to a point in the field.
Escape velocity (definition), and where is the equation derived from?
Min V required to escape gravitational field strength of an object (when projected vertically from the surface).
Derived by equation 1/2 mv^2 = GMm / r
Fc = Fg
If Fc = mv^2 / r
And Fg = -GMm / r^2
Then what can be stated?
m = rv^2 / G
What’s the equation for V (stable velocity)?
V = square root ( GM / r )
Equation for change in Grav potential energy;
Change Ep = m x g x change h
How do you prove the T^2 is directly proportional to r^3
T=d / v
T= 2pi x r / v
T^2 = 4pi^2 x r^2 / v^2
v^2 = MG / r
T^2 = 4pi^2 x r^3 / MG
