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A level physics > 18 - Gravitational Fields > Flashcards

18 - Gravitational Fields Flashcards

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1
Q

What objects create gravitational fields around them?

A

All objects with mass

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2
Q

How far does a gravitational field extend?

A

To infinity

Becomes infinitely small at long distances

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3
Q

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

A

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

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4
Q

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

A

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
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5
Q

What is the symbol for gravitational field strength?

A

g

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6
Q

Is gravitational field strength a vector?

A

Yes

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7
Q

Where does gravitational field strength always point?

A

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

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8
Q

Can gravitational field lines cross?

A

NO

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9
Q

Where do the arrows on gravitational field lines always point/

A

Towards the centre of mass of the object.

In the direction of the field.

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10
Q

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

A

Their density.

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11
Q

What do close gravitational field lines represent?

A

A strong gravitational field

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12
Q

What do far apart gravitational field lines represent?

A

A weak gravitational field.

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13
Q

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

A

A point mass

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14
Q

What does it mean if a gravitational field is uniform?

A

The field strength does not change.

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15
Q

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

A

Parallel and equally spaced lines.

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16
Q

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

A

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

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17
Q

What is Newton’s law of gravitation?

A

F = -(GMm) / r²

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18
Q

What is G?

A

The gravitational constant.

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19
Q

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

A

To show that the force is an attractive force.

It acts towards the object creating the field.

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20
Q

What is the separation, r, between?

A

The centre of the masses.

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21
Q

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

A

Add and subtract the forces

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22
Q

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

A

Use pythagoras / trigonometry to find the resultant force.

23
Q

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

A

The acceleration of free fall

9.81 ms-2 / Nkg-1

24
Q

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

A

Directly proportional

g ∝ M

25
Q

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

A

Inverse square

g ∝ 1 / r²

26
Q

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

A

g = -GM / r²

Use definition for gravitational field strength and Newton’s law of gravitation

27
Q

What is Kepler’s first law?

A

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

28
Q

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

A

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.

29
Q

What is Kepler’s second law?

A

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

30
Q

Do orbiting planets move at a constant speed?

A

NO

31
Q

Where does an orbiting planet move the fastest?

A

When it is closest to the sun

32
Q

When does an orbiting planet move the slowest?

A

When it is farthest from the Sun

33
Q

What is Kepler’s third law?

A

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

T² ∝ r³

34
Q

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

A

The gravitational force between it and the Sun

35
Q

What can Kepler’s laws be applied to?

A

Any object in orbit of another in the solar system

36
Q

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

A

Centripetal force = Gravitational force

Rearrange to find v

37
Q

What can satellites be used for?

A

Global positioning
Mapping
Reconaissance

38
Q

What is a geostationary satellite?

A

A satellite that never changes its position relative to the rotation of the Earth.

Orbits the equator with a period of 24 hours.

39
Q

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

A
  • Period of 24 hours
  • Orbit equator
  • Orbit in same direction as Earth’s rotation
40
Q

Define gravitational potential at a point in a gravitational field.

A

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

41
Q

What is the value of gravitational potential at infinity?

A

0

42
Q

What is the unit for gravitational potential?

A

J kg⁻¹

43
Q

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?

A

Inversely proportional

V ∝ 1 / r

44
Q

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

A

Directly proportional

V ∝ M

45
Q

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

A

The graph will tend towards 0.

46
Q

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

A

Straight line through 0/

47
Q

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

A

-GM

48
Q

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

A

It decreases.

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

49
Q

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

A

It increases

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

50
Q

Define gravitational potential energy.

A

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

51
Q

How do we calculate gravitational potential energy?

A

E = mV

= mass x gravitational potential

52
Q

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

A

The work done to move the object that distance.

W = Fx

53
Q

How does an object escape a planet’s orbit?

A

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

54
Q

What is escape velocity?

A

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

A level physics (26 decks)

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