Gravitational Fields F01-05

Question 1

Describe Newton’s law
of gravitation?

Answer 1

The force of attraction that is directly proportional to the product of the two masses, and inversely proportional to the distance between the two centres squared.

Question 2

Define ‘gravitational field strength’

Answer 2

The force per unit mass felt by a small test object at any point within that field.

Question 3

Is field strength a scalar or a vector?

Answer 3

Vector.

Question 4

Units of field strength?

Answer 4

N kg^-1

Question 5

Define ‘gravitational potential’

Answer 5

The work done PER UNIT MASS to bring an object from infinity to that point.

Question 6

Is potential a scalar or vector?

Answer 6

Scalar.

Question 7

Units of potential?

Answer 7

J kg^-1

Question 8

Draw the field lines in a radial field.

Answer 8

Circular centre with arrows all going towards to the centre of the circle.

Question 9

Draw the equipotentials in a radial field

Answer 9

A dotted circle around the circle centre in a circle shape.

Question 10

Draw the field lines in a uniform field.

Answer 10

Parallel lines pointing towards the ground/downwards.

Question 11

Draw the equipotentials in a uniform field.

Answer 11

Dotted lines perpendicular to the direction of the force field.

Question 12

How do the field lines indicate field strength?

Answer 12

The closer the lines are together the stronger the field is.

Question 13

What is the angle between equipotentials and field lines?

Answer 13

Perpendicular to the direction of the field lines so 90 degrees.

Question 14

What is the graph of the field strength against distance?

Answer 14

A typical y = 1/x graph

Question 15

What is the graph of the potential against distance?

Answer 15

r shaped curve starting in the bottom right quadrant away from the y axis, at the radius of sphere, ending at the surface of the next object or the total distance.

Question 16

How can we find the work done moving between points from a g vs r graph?

Answer 16

Work done is the area under the graph

Question 17

How can we find the field strength from a V vs r graph?

Answer 17

Finding the gradient = -(V/r)

Question 18

Show that T² ∝R³ (Kepler’s law)?

Answer 18

F = Gm1m2/r^2
F = mv^2/r
GMm/r^2 = mv^2/r
From above derive… v = (GM/r)^1/2
v = 2pir/T
T = 2pir/v
T = 2pir/(GM/r)^1/2
T = (4pi^2r^3/GM)^1/2
Therefore T^2 = 4pi^2r^3/GM
Therefore T² ∝R³

Question 19

Describe Newton’s law
of gravitation formula?

Answer 19

F = (Gm1m2)/(r^2)

Question 20

What is the value of the gravitational potential at infinity?

Answer 20

As r tends to infinity, 0.

Question 21

What are equipotential lines?

Answer 21

Definition: Join points of equal potential.
Orientation: at 90 degrees to the direction of the field lines.
Spacing: are closer in stronger in stronger fields.

Question 22

Is there work done along equipotential lines?

Answer 22

No work done is moving along an equipotential lines.

Question 23

What is a geostationary/geosynchronous satellite?

Answer 23

Time period is 24 hours.
Stays above the same point on the earth.
Cannot see the poles, (only up to 70 degrees above the equator).

Question 24

What is a Low orbit (Polar) satellite?

Answer 24

Over the poles.
This way a satellite can cover the whole earth as the earth rotates.
Short time period, around 90 minutes.

Question 25

How to calculate the total GFS?

Answer 25

g(T) = g(1) - g(2)
g(1) > g(2)

Question 26

How to calculate the total Gravitational Potential?

Answer 26

V(T) = V(1) + V(2)

Question 27

What is the equation relating work done, mass and gravitational potential?

Answer 27

W = mV
W = m(V(orbit) - V(surface))

Question 28

What are the uses for geostationary/geosynchronous satellite?

Answer 28

Communications

Question 29

What are the uses for Low orbit (Polar) satellite?

Answer 29

Military
Scientific
Weather
GPS

Question 30

What are the energy considerations for an object in orbit?

Answer 30

Total energy = KE + GPE (negative)
If total < 0, then stays in orbit.
If total > 0, then will escape orbit.
(KE > GPE)