Gravitiational Fields

Question 1

What is a gravitational force

Answer 1

A force of attraction that acts between objects with mass

Question 2

Define Newton’s Law of Gravitation

Answer 2

The gravitational force between 2 masses is directly proportional to the product of the masses and inversely proportional to the square of the distance between them

Question 3

What is a uniform gravitational field

Answer 3

A field that exerts the same gravitational force on a mass anywhere in the field

Question 4

What is a radial field

Answer 4

A field in which the force exerted depends on an objects position in the field. Decreases as the distance from the centre increases

Question 5

Define Gravitational Field Strength

Answer 5

The force per unit mass exerted by a gravitational field on an object.

Question 6

How does Gravitational field strength vary in a uniform and radial field

Answer 6

Uniform - Always the same Radial - Varies with distance

Question 7

Define Gravitational potential at a point

Answer 7

The work done per unit mass when moving an object from infinity to that point.

Question 8

Why is Gravitational potential always negative

Answer 8

Gravitational potential at infinity is zero, and as an object moves from infinity to a point, energy is released as the gravitational potential energy is reduced

Question 9

Define Gravitational Potential Difference

Answer 9

The energy needed to move a unit mass between to points

Question 10

What is an equipotential surface

Answer 10

Surfaces which are created through joining points of equal potential together, therefore the potential on an equipotential surface is constant everywhere.

Question 11

What is the relationship between gravitational potential and distance between the centre of masses

Answer 11

Question 12

What is the gradient of a V-r graph

Answer 12

Gravitational field strength x -1

Question 13

What is the area under a g-r graph

Answer 13

Gravitational potential difference

Question 14

What is Keplers 3rd Law and the equation

Answer 14

T^{<strong>2</strong>} is proportional to r³

T² = r³ x 4π²/GM

Question 15

Describe the context that Keplers 3rd law should be used

Answer 15

An object orbiting a body

M = Mass of the body being orbited

r = Dist. between the object and the centre of the body being orbitted

Question 16

What is the derivation for Keplers 3rd Law

Answer 16

When an object orbits a mass in circular motion, the gravitational force acts as the centripetal (resultant) force

mv²/r = GMm/r² , mv² = GMm/r , v²=GM/r

Since v=d/t, let d be the circumference = 2πr

v=2πr/T , v²=4π²r²/T²

Equate the 2 equations

4π²r²/T²=GM/r

T²=4π²r³/GM

Question 17

What is the total energy of a satellite in orbit

Answer 17

Sum of Kinetic Energy and Potential Energy

Question 18

How do the energy stores in an orbiting satellite change

Answer 18

Total energy is constant

i.e If it orbits at a lower height, Potential energy falls, so Kinetic energy increases to keep total energy constant

Question 19

What is escape velocity

Answer 19

The minimum velocity an object must travel at in order to escape the gravitational field at the surface of a mass, when it is projected vertically from the surface

When E_k=E_p

As E_p is the energy required for an object to leave a field, so when its E_k is high enough, it has enough energy to transfer.

Question 20

How to calculate escape velocity

Answer 20

When E_k=E_p

v = (2GM/r)^½

M= Mass of larger mass

Question 21

What is a synchronous orbit

Answer 21

Where the orbital period is equal to the rotational period of the object it’s orbitting

Question 22

What is a geostationary Satellite

What can they be used for

Answer 22

A satellite that follows a geosynchronous orbit

Always stays above the same point as they orbit directly above the equator

Used in TV/telephone signals as its position never changes

Question 23

What is a low orbit satellite, what are they used for

Answer 23

A satellite with a much lower and faster orbit

Can orbit the entire earth

Used for military, weather, scientific purposes

Question 24

What is the relationship between Gravitational Field Strength and Potential Gradient

Answer 24

Gravitational Field Strength = -Potential Gradient

As potential gradient is how much energy a 1kg mass would transfer to move 1m further. This is the work done against the attractive gravitational force, which is facing the opposite direction.

That attractive gravitational force for the 1kg mass is just the gravitational field strength