Gravitational Fields

Question 1

Newton’s Law of Gravitation

Answer 1

The gravitational force of attraction between two masses is directly proportional to the product of their masses and inversely proportional to the square of the separation of their COM’s

Question 2

Gravitational field strength at a point

Answer 2

The force acting per unit mass at that point

Question 3

Field lines in a uniform field are…

Answer 3

Parallel and equally spaced

Question 4

Satellite

Answer 4

An object that moves around a planet/star at a constant speed (but is constantly accelerating)

Question 5

Centripetal force for satellites

Answer 5

Gravitational force

Question 6

What is Kepler’s 3rd law derived from

Answer 6

Centripetal force = Gravitational force

Question 7

Kepler’s 3rd law

Answer 7

T²∝r³

Question 8

Why are there two formulae for GPE?

Answer 8

GPE=mgh is only valid for small distances

Question 9

Gravitational potential at a point

Answer 9

The work done in bringing a unit mass from infinity to that point

Question 10

Why is gravitational potential negative?

Answer 10

• V = 0 at infinity
• Objects lose energy as they enter a field as the field does work against them

Question 11

Does the work done by a field depend on the route through the field?

Answer 11

No

Question 12

Relationship between V and r

Answer 12

Inverse

Question 13

Relationship between F (or g) and r in a radial field

Answer 13

Inverse square

Question 14

Gravitational equipotential

Answer 14

A set of points that all have the same potential (no work is done to move along an equipotential)

Question 15

How can gravitational field strength be described?

Answer 15

The potential gradient

Question 16

How does energy of a satellite in an elliptical orbit change?

Answer 16

• The sum of KE and GPE is constant
• GPE is transferred to KE as the satellite moves closer to the object it is orbiting

Question 17

Higher orbits have _____ energy than lower orbits

Answer 17

more

Question 18

How is the equation for escape velocity derived?

Answer 18

Energy conservation

Question 19

What does r represent in the escape velocity equation(v = sqrt(2GM/r)?

Answer 19

The initial radial distance of the object escaping (usually the radius of the planet)

Question 20

Synchronous orbit

Answer 20

Satellite has the same orbital period as the rotational period of the planet

Question 21

Geostationary satellites

Answer 21

• Have an orbital period of 24hrs
• Orbit in the plane of the equator
• Stays above a fixed point on the earth

Question 22

Polar satellites

Answer 22

• Obit in a north-south plane
• Cover a large area as the earth rotates through their orbit

Question 23

Uses of low orbit satallites

Answer 23

Weather mapping and spying (can see the earths surface in detail)

Question 24

Uses of geostationary satellites

Answer 24

Communications (GPS and TV signals)

Question 25

How can gravitational fields be combined?

Answer 25

They are vector fields so can be added (but the directions will also be affected)