Gravitational Fields

Question 1

What does Newton’s Law of Gravitation state?

Answer 1

The gravitational force between two point masses is proportional to 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/r²

Question 3

What does ‘G’ represent?

Answer 3

Gravitational Constant. Equal to 6.67x10⁻¹¹ Nm ²kg⁻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 experiences 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

N kg⁻¹

Question 8

State the equation for gravitational field strength.

Answer 8

g = Gm/r²
g = F/m

Question 9

What is the gravitational potential at a point?

Answer 9

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

Question 11

State the equation for gravitational potential energy.

Answer 11

GPE = mass x gravitational field strength x height
= -GMm/r

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

Question 14

What does the area under a force-separation graph represent?
(Gravitational Fields)

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

The difference in gravitational potentials between 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 inside the field to outside the field. Since outside the field, the potential is defined as zero, then the potential inside the field must be zero.

Question 22

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

Answer 22

T² ∝ R³ (kepler’s third law)

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 which has a greater r

Question 24

What is the period of a geosynchronous orbit?

Answer 24

24 hours - 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 and vice versa

Question 28

Describe similarities between electric and gravitational fields.

Answer 28

• forces both follow an inverse square law
• use field lines to be represented and can both be uniform or radial ( Gravitational never fully radial but for example near to the Earth’s surface its pretty much uniform)
• Similar equations to find force exerted and field strength

Question 29

What are differences between gravitational and electric fields?

Answer 29

• In gravitational fields, the force exerted is always attractive while in electric fields it can be attractive or repulsive
• Electric fields act on charge, while gravitational fields act on mass (all objects)

Question 30

How can you derive kepler’s third law using equations, this is for an object orbiting a mass?

Answer 30

Centripetal force = mv²/r
Gravitational force = GMm/r²
Equate them and so v²= GM/r
But v = 2πr/T
v² =4π²r²/T² = GM/r
T² = 4π²/ GM x r³