Year 2 Chapter 3: Gravitation

Question 1

What is gravity?

Answer 1

A universal attractive force acting between all matter

Question 2

What is a force field?

Answer 2

An area in which an object will experience a non-contact force

Question 3

What is Newton’s Law of Gravitation?

Answer 3

F=GMm / r^2

Question 4

What is G?

Answer 4

The universal constant of gravitation
6.67 x 10^-11 N m^2 kg^-2

Question 5

What is a gravitational field?

Answer 5

A region surrounding a mass in which any other object with mass will experience an attractive force

Question 6

What is gravitational field strength and what is it measured in?

Answer 6

The attractive gravitational force per unit mass exerted on a small test mass placed at a point in that field, measured in N / kg

Question 7

What can gravitational fields be represented by and what do they show?

Answer 7

Field lines which represent the direction and strength of the field, showing the path that a test mass would take when placed within the gravitational field at that point

Question 8

What are some features of field lines?

Answer 8

Point towards centre of mass producing the field
Show direction test mass moves…
Closeness represents field strength
Never cross over each other

Question 9

What is a uniform field?

Answer 9

A field in which all of the field lines are parallel and equally spaced – field strength is equal in all areas of the field, eg gravity on earth’s surface

Question 10

What is a radial field?

Answer 10

A field in which the field lines are all directed towards a single point, eg the centre of a planet

Question 11

What is an equation for g for all fields?

Answer 11

g = F/m

Question 12

What is an equation for g in radial fields?

Answer 12

g = GM/ r^2

Question 13

What is the gravitational field strength on the surface of the earth?

Answer 13

9.81 N / kg

Question 14

What is gravitational potential, V?

Answer 14

The work done per unit mass required to move a small test mass from infinity to a given point in a field, measured in J / kg

Question 15

What is the equation for gravitational potential?

Answer 15

ΔV = gΔh

Question 16

Why is gravitational potential negative?

Answer 16

Gravitational potential is defined as being zero at infinity and as it decreases as it moves towards the centre of the field it is always a negative value

Question 17

What is the equation for work done when moving a mass in a gravitational field?

Answer 17

ΔW = mΔV

Question 18

What is gravitational potential energy?

Answer 18

The component of an object’s energy due to its position in a gravitational field

Question 19

What is an equipotential surface?

Answer 19

A surface of constant potential. No work is done by the field when an object moves along an equipotential.

Question 20

What is a feature of equipotential lines relating to field lines?

Answer 20

They are perpendicular to field lines

Question 21

What is a potential gradient?

Answer 21

The change of potential per metre at a point in the field

Question 22

What is the equation for gravitational potential gradient and why is it negative?

Answer 22

g = -ΔV / Δh or g = -ΔV / Δr
Potential gradient is positive direction upwards, field direction is downwards

Question 23

What is the equation for V in a radial field?

Answer 23

V = -GM / r

Question 24

How do you find ΔV from a graph of g against r?

Answer 24

Find the area under the curve

Question 25

What is Kepler’s Third Law?

Answer 25

T^2 α R^3

Question 26

What is escape velocity?

Answer 26

The minimum velocity required by an object to be able to escape a gravitational field of a mass when projected vertically from its surface

Question 27

How can escape velocity be calculated?

Answer 27

Ek = mΔV = GMm/r

Question 28

What is the centripetal force in orbits?

Answer 28

Gravitational force, resulting in circular motion

Question 29

What is a synchronous orbit?

Answer 29

An orbit in which the period of the orbit is equal to the rotational period of the object that it is orbiting.

Question 30

What is a geostationary satellite?

Answer 30

A satellite that orbits above the equator with a 24 hour period, so it will always remain above the same position on the Earth. They orbit approximately 36,000km above the surface of the Earth.