12 - gravitational fields

Question 1

What does Newton’s Law of Gravitation state?

Answer 1

The gravitational force between two point masses is proportional to the product of their masses and is inversely proportional to the square of the separation of their centres.

Question 2

State the defining equation for Newton’s Law of
Gravitation.

Answer 2

F = -GMm / r^2
r is the seperation from the centre of the earth

Question 3

What does ‘G’ represent?

Answer 3

The universal gravitational constant, which is equal to 6.67 x 10^-11 N m2 kg-2

Question 4

What is always true about the gravitational force between two masses?

Answer 4

It is always an attractive force.

Question 5

What is a gravitational field?

Answer 5

A region in which any object with mass will experience a non-contact force.

Question 6

What is gravitational field strength?

Answer 6

The force per unit mass felt by the object.

Question 7

What is the unit for gravitational field strength?

Answer 7

Nkg^-1

Question 8

State the equation for gravitational field strength.

Answer 8

g = GM / r^2
or
g = F / m

r is seperation from earth centre

Question 9

What is gravitational potential at a point?

Answer 9

The energy transferred per unit mass when moving an object from infinity to that point.

Question 10

State the equation for gravitational potential.

Answer 10

V = -GM / r
r is seperation from earths centre

Question 11

State the equation for gravitational potential energy.

Answer 11

E = -GMm / r

r is seperation from earths centre

Question 12

What is an equipotential?

Answer 12

A plane in which all points have the same potential.

Question 13

What is true when a mass moves along an equipotential?

Answer 13

No work is done when moving along an equipotential.

Question 14

What does the area under a force-separation graph represent?

Answer 14

Energy

Question 15

What is gravity?

Answer 15

Gravity is the universal attractive force which acts between all matter.

Question 16

What can field lines tell you about a field?

Answer 16

The direction of the arrows show the direction of the field. The strength of the field is represented by the density of the field lines.

Question 17

What is gravitational potential?

Answer 17

The potential energy per unit mass, at any point in the field. Zero potential is defined at infinity, hence at a point close to a mass the potential of an object would be negative.

Question 18

How do you calculate the work done by moving a mass in a field?

Answer 18

Work Done = Mass x Change in
Gravitational Potential

Question 19

What is gravitational potential difference?

Answer 19

Gravitational potential difference is the difference in the gravitational potentials of two points in a gravitational field.

Question 20

How much work is done per unit distance when you move along an equipotential?

Answer 20

No work is done when moving across equipotentials, as the potential at each point is the same.

Question 21

Why is gravitational potential a negative value?

Answer 21

Work needs to be done to move an object from the inside the field to outside the field.
Since outside the field’s potential is defined as zero then the potential inside the field must be negative.

Question 22

How is the orbital period related to the radius of a circular orbit?

Answer 22

T^2 ∝ R^3

Question 23

Compare the PE and KE of a lower orbit to a higher one.

Answer 23

A lower orbit (smaller m) has less potential energy and more kinetic energy than a higher orbit (bigger r).

Question 24

What is the period of a geosynchronous orbit?

Answer 24

Geosynchronous orbits have a period of one day.

Question 25

What type of matter is affected by a gravitational field?

Answer 25

Any object or matter with mass will experience an attractive force.

Question 26

What is a point mass?

Answer 26

A point mass is a theoretical object that has a mass but no dimensions. This means that all of its mass acts at a single point.

Question 27

What is the relationship between the field strength and the field lines that represent it?

Answer 27

The closer the lines are together, the stronger the gravitational field strength.
The further apart the lines are, the weaker the field strength.