18 - Gravitational Fields

Question 1

What objects create gravitational fields around them?

Answer 1

All objects with mass

Question 2

How far does a gravitational field extend?

Answer 2

To infinity

Becomes infinitely small at long distances

Question 3

What will happen to any object with mass that is placed inside a gravitational field?

Answer 3

It will experience an attractive force towards the centre of mass of the object creating the field.

Question 4

Define GRAVITATIONAL FIELD STRENGTH at a point within a gravitational field.

Answer 4

The gravitational force exerted per unit mass on a small object placed at that point within the field.

g = F / m

F = Gravitational force
m = mass of object in field

Question 5

What is the symbol for gravitational field strength?

Answer 5

g

Question 6

Is gravitational field strength a vector?

Answer 6

Yes

Question 7

Where does gravitational field strength always point?

Answer 7

Towards the centre of mass of the object creating the field.

Question 8

Can gravitational field lines cross?

Answer 8

NO

Question 9

Where do the arrows on gravitational field lines always point/

Answer 9

Towards the centre of mass of the object.

In the direction of the field.

Question 10

How is the strength of a gravitational field represented by field lines?

Answer 10

Their density.

Question 11

What do close gravitational field lines represent?

Answer 11

A strong gravitational field

Question 12

What do far apart gravitational field lines represent?

Answer 12

A weak gravitational field.

Question 13

What do we model every object which is creating a gravitational field as?

Answer 13

A point mass

Question 14

What does it mean if a gravitational field is uniform?

Answer 14

The field strength does not change.

Question 15

How do we represent a uniform gravitational field using field lines?

Answer 15

Parallel and equally spaced lines.

Question 16

What does Newton’s law of gravitation state (in words)?

Answer 16

A point mass will attract another point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers

Question 17

What is Newton’s law of gravitation?

Answer 17

F = -(GMm) / r²

Question 18

What is G?

Answer 18

The gravitational constant.

Question 19

Why does Newton’s law of gravitation use a minus sign?

Answer 19

To show that the force is an attractive force.

It acts towards the object creating the field.

Question 20

What is the separation, r, between?

Answer 20

The centre of the masses.

Question 21

How do we calculate the force between multiple objects that are in the same plane?

Answer 21

Add and subtract the forces

Question 22

How do we calculate the force between multiple objects that are in different planes?

Answer 22

Use pythagoras / trigonometry to find the resultant force.

Question 23

What is the gravitational field strength of the Earth equal to, close to the surface of the earth?

Answer 23

The acceleration of free fall

9.81 ms-2 / Nkg-1

Question 24

What is the relationship between the gravitational field strength at a point and the mass of the object creating the field?

Answer 24

Directly proportional

g ∝ M

Question 25

What is the relationship between the gravitational field strength at a point and the distance between the centre of mass and the object?

Answer 25

Inverse square g ∝ 1 / r²

Question 26

How do we calculate the gravitational field strength at a point?

Answer 26

g = -GM / r² Use definition for gravitational field strength and Newton's law of gravitation

Question 27

What is Kepler's first law?

Answer 27

The orbit of a planet is an ellipse with the Sun as one of its two foci.

Question 28

Why do we often model the orbit of a planet as a circle if it is actually an ellipse?

Answer 28

Because there is such as small difference between the length of the sides. The longer side is very similiar to the shorter side. Low eccentricity.

Question 29

What is Kepler's second law?

Answer 29

A line segment joining the sun and the orbiting planet sweeps out equal areas in equal time intervals.

Question 30

Do orbiting planets move at a constant speed?

Answer 30

NO

Question 31

Where does an orbiting planet move the fastest?

Answer 31

When it is closest to the sun

Question 32

When does an orbiting planet move the slowest?

Answer 32

When it is farthest from the Sun

Question 33

What is Kepler's third law?

Answer 33

The square of the orbital period, T, is directly proportional to the cube of its average distance, r, from the Sun. T² ∝ r³

Question 34

What provides the centripetal force required to keep a planet in orbit?

Answer 34

The gravitational force between it and the Sun

Question 35

What can Kepler's laws be applied to?

Answer 35

Any object in orbit of another in the solar system

Question 36

What two equations do we use to calculate the orbital speed of a satellite?

Answer 36

Centripetal force = Gravitational force Rearrange to find v

Question 37

What can satellites be used for?

Answer 37

Global positioning Mapping Reconaissance

Question 38

What is a geostationary satellite?

Answer 38

A satellite that never changes its position relative to the rotation of the Earth. Orbits the equator with a period of 24 hours.

Question 39

What three criteria must a satellite fulfill in order to be a geostationary satellite?

Answer 39

- Period of 24 hours - Orbit equator - Orbit in same direction as Earth's rotation

Question 40

Define gravitational potential at a point in a gravitational field.

Answer 40

The work done per unit mass to bring an object from infinity to that point.

Question 41

What is the value of gravitational potential at infinity?

Answer 41

0

Question 42

What is the unit for gravitational potential?

Answer 42

J kg⁻¹

Question 43

What is the relationship between gravitational potential at a point in a gravitational field and the distance between the object creating the field and the point?

Answer 43

Inversely proportional V ∝ 1 / r

Question 44

What is the relationship between gravitational potential at a point in a gravitational field and mass of the object creating the field?

Answer 44

Directly proportional V ∝ M

Question 45

What will a graph of V against r show as r approaches infinity?

Answer 45

The graph will tend towards 0.

Question 46

What will the graph of V against 1/r look like?

Answer 46

Straight line through 0/

Question 47

What will the gradient of a graph of V against 1/r be?

Answer 47

-GM

Question 48

How does the gravitational potential change as you approach a point mass creating a field?

Answer 48

It decreases. | Negative number to more negative number (e.g. -40 to -80)

Question 49

How does the gravitational potential change as you move away from a point mass creating a field?

Answer 49

It increases | Negative number to less negative number (e.g. -40 to -20)

Question 50

Define gravitational potential energy.

Answer 50

The work done to move a mass from infinity to a point in a field.

Question 51

How do we calculate gravitational potential energy?

Answer 51

E = mV = mass x gravitational potential

Question 52

On a graph of gravitational force against distance, r, what does the area under the graph represent?

Answer 52

The work done to move the object that distance. W = Fx

Question 53

How does an object escape a planet's orbit?

Answer 53

It must have enough kinetic energy to overcome the gravitational energy to move an object out of the field.

Question 54

What is escape velocity?

Answer 54

The minimum velocity that any object must be travelling at to have enough kinetic energy to overcome the gravitational force and break orbit.