C18 (Gravitational Fields)

Question 1

Gravity

Answer 1

Universal attraction between any two masses
(Relatively weak force, only noticeable when one of masses very large, but also present tiny scale)

Question 2

What is the gravitational pull excreted on a n object directly proportional to?

Answer 2

The mass

Question 3

Weight

Answer 3

Force due to gravity

Question 4

Gravitational force (equation)

Answer 4

Fg=W

Question 5

Gravitational Field

Answer 5

Vol that gravity will act in

Question 6

Gravitational field strengths equal to what:

Answer 6

Gravity
W=mg

Question 7

Gravitational potential energy

Answer 7

Change GPE= mg x change h

Question 8

‘G’-Universal

Answer 8

Same throughout universe
G, very small, only significant when at least on mass is very large

Question 9

Why does the gravity equation contain a negative sign?

Answer 9

Indicates that force is attractive

Question 10

What does the lines in the gravitational field indicate?

Answer 10

Direction of Fgrav would act on test mass

Question 11

Test mass

Answer 11

Mass, small enough to not have no affect on shape of field with its own gravity

Question 12

The more lines per area (gravitational field):

Answer 12

The stronger the field

Question 13

How can you detect very small changes in gravity

Answer 13

Gravimeter
Used map tunnels, caves, change volcanic activity and minerals for mining

Question 14

For object moving in a circle, what’s it’s resultant force called

Answer 14

The centripetal force

Question 15

Centripetal force provided by…

Answer 15

Gravitational attraction of M (large body)

Question 16

Centripetal force is equal to…

Answer 16

Gravitational force
Fc=Fg

Question 17

Kepler’s first law

Answer 17

The orbit of every planet is an ellipse with the sun at one of the foci.

Question 18

Perihelion

Answer 18

Closet distance to sun

Question 19

Aphelion

Answer 19

Furthest distance from sun

Question 20

Eccentricity

Answer 20

Measure of how elongated the circle is (usually orbits have low eccentricity).

Question 21

Kepler’s second law

Answer 21

A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time

Question 22

As planets move on their elliptical orbit their speed….

Answer 22

Isn’t constant
When closer they travel faster

Question 23

Kepler’s third law

Answer 23

The square of the orbital period (T) of a planet is directly proportional to the cube of its average distance (r) from the Sun

Question 24

The gradient of a graph with T squared against r cubed is…

Answer 24

4Pi^2 / GM

Question 25

Satellite

Answer 25

Body that circles round another body in space

Question 26

Give examples of natural satellites

Answer 26

Moons
Stars
Comets
Asteroids
Planets

Question 27

Uses of satellites

Answer 27

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

Question 28

Stallites orbiting Earth obey what laws

Answer 28

Kepler’s laws

Question 29

What provides the centripetal force on each satellite

Answer 29

Gravitational force between it and the Earth

Question 30

Why don’t the satellites fall towards earth if the only force acting upon them is Fg

Answer 30

Always falling,however travels great distance, that as it falls Earth curves away beneath it, keeping it the same height.

Question 31

What is required for a stable orbit

Answer 31

Exactly right height and speed

Question 32

The correct speed for stable orbit at distance r from centre mass of Earth is given by…

Answer 32

V=✓Gm/r

Question 33

What does every satellite that is placed in a given orbit at a given height have in common

Answer 33

They all travel at the same speed (even if their masses vary)

Question 34

Once launched they’re normally above atmosphere, where there’s no air resistance meaning…

Answer 34

No air resistance to slow them down, as a result they travel at a constant speed

Question 35

Polar orbits

Answer 35

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

Question 36

Low Orbit

Answer 36

Satellites in orbit close to Earth, Kepler’s third law (T squared directly proportional to r cubed)
Only short time taken to orbit Earth

Question 37

Geostationary satellites

Answer 37

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

Question 38

To be in a geostationary orbit satellite must…

Answer 38

Be in orbit above Equator
Rotate same direction as Earth
Have orbital period, 24hrs

Question 39

The orbital period of geostationary satellites is how long?

Answer 39

24hrs

Question 40

Gravitational potential (definition):

Answer 40

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.

Question 41

Gravitational potential energy (definition):

Answer 41

The work done per unit mass in moving object from infinity to a point in the field.

Question 42

Escape velocity (definition), and where is the equation derived from?

Answer 42

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

Question 43

Fc = Fg
If Fc = mv^2 / r
And Fg = -GMm / r^2
Then what can be stated?

Answer 43

m = rv^2 / G

Question 44

What’s the equation for V (stable velocity)?

Answer 44

V = square root ( GM / r )

Question 45

Equation for change in Grav potential energy;

Answer 45

Change Ep = m x g x change h

Question 46

How do you prove the T^2 is directly proportional to r^3

Answer 46

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