18 : Gravitational fields

Question 1

What is gravity

Answer 1

Gravity is the universal attractive fore which acts between all matter

Question 2

What is G

Answer 2

The universal gravitational constant

Question 3

What can field lines tell you

Answer 3

The direction and strength of the field depending 9n the density of the field lines

Question 4

What is g

Answer 4

• his the force per unit area in a uniform gravitational field
• in a radial field the magnitude of g is the proportionality constant at that between force and mass
• g = GM/r^2

Question 5

Newtons law of gravitation

Answer 5

States that two point masses attract each other with a force that is directly proportional to the product of their masses and inversely proportional too the square of the distance between them

Question 6

Keplers first law

Answer 6

The orbit of a plant is an ellipse with the sun at one of the foci- the eccentricity f the ellipse is very low so the notion can be modelled as circular

Question 7

Keplers second law

Answer 7

A line segment joining a planet and the sun sweeps out equal areas during intervals of equal time
- the speed of the planet isn’t constant as it moves faster closer to the sun

Question 8

Keplers third law

Answer 8

The square of the orbital period T is proportional to the cube of the average distance r from the sun
- this can be proved with forces acting on planet - centripetal force is required to keep planet in orbit and this force is provided by gravitational field of the sun

Question 9

Derive equation to show T^2 is proportional to r^3

Answer 9

• Keplers third law = mv^2/r = GMm/r
• rearrange to get v^2 = GM/r
• velocity in circular motion is 2πr/T
• substitute in and rearrange to get T^2 as subject

Question 10

What are satellites

Answer 10

Are objects that orbit other, large objects these include natural satellites such as moons and articulated satellites that humans have sent into space

Question 11

Uses of satellites

Answer 11

Communications, scientific research and global positioning systems

Question 12

What are Geostationary satellites

Answer 12

Have an orbital period that is exactly a day so day appear stationary above the earth
- orbit 36000km above the equator

Question 13

Uses of geostationary satellites

Answer 13

Communications and surveying as the provide continuous coverage

Question 14

Gravitational potential

Answer 14

The potential energy per kilogram, at any point in the field
- 0 potential is defined at infinity as at a point close to a mass the potential energy is negative as its an attractive force

Question 15

What is Gravitational potential difference

Answer 15

The difference in gravitational potentials of two points in a gravitational field

Question 16

What is gravitational potential energy at a point in the field

Answer 16

The work done per unit mass is moving object from infinity to that point in the field

Question 17

What is escape velocity

Answer 17

The minimum velocity an object requires in order to escape the gravitational field of an object when projected vertically form its surface
- formula derived from equating kinetic energy and gravitational potential energy required to reach infinity
- 1/2 mv^2 = GMm/r