Gravitational Fields

Question 1

What is Newton’s Universal Law of Gravitation?

Answer 1

The force of attraction between two point masses is
Proportional to the product of their masses
Inversely proportional to the square of their distance apart

Question 2

Where must radial distances be measured from?

Answer 2

The centre of mass

Question 3

What is meant by “Point Masses”?

Answer 3

It is assumed that the masses are infinitesimally small

Question 4

What is the equation for Newton’s Law of Gravitation?

Answer 4

F=GMm/r²

Question 5

What is the Universal Gravitational constant?

Answer 5

6.67x10^-11

Question 6

When a question refers to the height of a satellite, what is meant?

Answer 6

The height of the satellite from Earth’s surface.

Therefore r=radius of earth + height above Earth’s surface

Question 7

What is the definition of Gravitational Field Strength?

Answer 7

The Force acting on a unit mass

Question 8

What is the equation of the definition of Gravitational Field Strength?

Answer 8

g=F/m

Question 9

What equation is used to calculate Gravitational Field Strength?

Answer 9

g=GM/r²

Question 10

Is a g field a scalar or vector?

Answer 10

Vector

Question 11

What is the direction of a g field?

Answer 11

Always towards the centre of mass

Question 12

What does the magnitude of the g field depend on?

Answer 12

The mass of the body creating the field

Question 13

Where is the g field greatest?

Answer 13

At the surface of the body creating it

Question 14

What happens to the g field within the body creating it?

Answer 14

It reduces to zero as you approach its centre

Question 15

What is Kepler’s Third Law of Planetary Motion

Answer 15

T²=4π²r³/GM

Question 16

What provides the centripetal force required for Earth to orbit the Sun?

Answer 16

The gravitational force of attraction.

Question 17

For an orbit to be Geostationary, what must it do?

Answer 17

Have a period of 24 hours
Lie on Earth’s equatorial plane
Rotate the same direction of Earth

Question 18

What is an advantage of a Geostationary orbit?

Answer 18

The satellite appears stationary to the observer, therefore doesnt require tracking
The satellite will not set beneath the observers horizon.

Question 19

How many Geostationary satelittes are required of full global coverage?

Answer 19

3

Question 20

What is the equation for Gravitational Potential Energy?

Answer 20

GPE=-GMm/r

not on equation sheet

Question 21

What is the definition of Gravitational Potential Energy?

Answer 21

The work done in moving a mass form infinity to a distance r within a gravitational field.

Question 22

Why are all GPE’s negative?

Answer 22

Energy is required to move a mass to infinity.
At infinity the field is zero, therefore the force is zero, therefore the GPE is zero.
At a distance less than infinity energy is lost therefore GPE is zero.

Question 23

What equation is used to calculate the change in GPE?

Answer 23

ΔGPE= - Gmm (1/r(2) - 1/r(1) )

Question 24

What is the equation for escape velocity?

Answer 24

v²=2GM/r

Question 25

What equations are used in deriving the equation for escape velocity?

Answer 25

W=0.5mv²
and
GPE=GMm/r

Question 26

What is escape velocity?

Answer 26

The minimum velocity required to leave a body and travel infinetly far away.

Question 27

Rockets to orbit do not achive escape velocity, why is this?

Answer 27

Escape velocity is to get infinetly far away.

Orbits are not infinetly far away.

Question 28

A rockets initial launch velocity is not escape velocity, why is this?

Answer 28

Rockets accelerate over a longer period. As escape velocity decreases with altitude as the field strength decreases.

Question 29

Define Gravitational Potential

Answer 29

The work done in moving a unit mass from infinity to a distance r within the gravitaional field

Question 30

what is the unit of Graviational Potential?

Answer 30

Joules per Kilogram

JKg^-1

Question 31

What is the equation to calculate Gravitational Potential?

Answer 31

Vg=-GM/r
Vg is subscrpt g

(As Gravitational Potentials is the same as Gravitational potential energys but for a unit mass, imagine
GMm/r with m=1 as it is a unit mass)

Question 32

Is Gravitational Potential scalar or vector?

Answer 32

Scalar

Question 33

Are Gravitataional Potentials positive or negative?

Answer 33

Negative

Question 34

The magnitude of Gravitational Potentails relies on what?

Answer 34

Mass of the body creating the field

Question 35

What is Gravitational Potential proportional to?

Answer 35

1/r

Question 36

Where is Gravitational Potential greatest?

Answer 36

At the surface of body creating the field

Question 37

What equation is used to calculate a change in Gravitational Potential?

Answer 37

ΔV=-Gm(1/r(2) - 1/r(1))

Question 38

What equation links Work done, Mass and Gravitational Potential?

Answer 38

ΔW=mΔV

Question 39

What is a Gravitational Equipotential?

Answer 39

A graphical means to display Gravitaional Potential

They are perpendicular to field lines

Question 40

Are Equipotentials equally spaced in radial and uniform fields?

Answer 40

In uniform fields equipotentials are equally spaced as the field strength doesn’t change.
In radial fields equipotentials are not equally spaced as the field strength weakens, V is proportional to 1/r.

Question 41

What is the equation related to Potential Gradients?

Answer 41

g= - ΔV/Δr

Question 42

What is the Potential Gradient equal to?

Answer 42

The field strength at that point in space

Question 43

What is the gradient of a distance-gravitational potential graph equal to?

Answer 43

Field strength at that point in space