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Physics alevel ocr a > Chapter 18 gravitational fields > Flashcards

Chapter 18 gravitational fields Flashcards

1
Q

What are gravitational foibles due to?

A

Mass

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2
Q

How is the mass of a spherical object modelled?

A

As a point at the centre of mass

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3
Q

What do all objects with mass create?

A

A gravitational field

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4
Q

Where do gravitational fields extend to?

A

Infinity (but gets weaker as distance from centre of mass increases, becoming negligible at long distances )

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5
Q

What will any object with mass experience in a gravitational field?

A

An attractive force towards the centre of mass of the object creating the field (the object with the larger mass out of the 2 masses)

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6
Q

What is the gravitational field strength equal to on earth?

A

9.81ms^-2 (free fall)

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7
Q

Is gravitational force always attractive?

A

Yes

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8
Q

How do we represent a stronger gravitational field with field lines?

A

The field lines are closer together

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9
Q

What do field lines form around a spherical mass?

A

A radial field

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10
Q

What properties do field lines have if there is a uniform gravitational field?

A

They are:
-equidistant ( equal distance between the field lines)
-parallel

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11
Q

Does the gravitational field strength change in a uniform field?

A

No

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12
Q

What are the gravitational field lines close to the surface of a planet?

A

Approximately uniform (simply straight lines towards the centre of mass which are parallel and equidistant, as when you get close enough to a planets surface the surface simply becomes flat)

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13
Q

What does Newtons law of gravitation state?

A

That the force between 2 masses is:
-directly proportional to the of the masses( F proportional to Mm)
[larger M is the mass that attracts the other mass i.e. has a larger mass and m is the mass that experiences the gravitational attraction from M]
-inversely proportional to the square of their separation (F is proportional to 1/r^2)

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14
Q

What does the minus sign represent in the equation for newtons law of gravitation?

A

It shows that the gravitational force is an attractive force.

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15
Q

How do i find the resultant force of an object, or multiple objects?

A

Model them as vectors and use vector addition and subtraction while some cases may require you to use a force diagram.

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16
Q

How do you derive the equation for gravitational field strength in a radial field?

A

Use g=F/m And F =-GMm/r^2 Sub F

Get g=(-GMm/r^2)/m The m’s cancel
Get g=-GM/r^2

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16
Q

What does the negative sign represent in the derived equation for gravitational field strength in a radial field?

A

It shows that the gravitational field strength at that point is in the opposite direction to the displacement r from the centre of mass( a gravitational field is an attractive field)

17
Q

What is Keplers first law?

A

The orbit of a planet is an ellipse with the sun at one of 2 foci points ( foci = the two bits towards the end of an ellipse but still in the middleish)

18
Q

What is the aphelion?

A

The furthest point from the sun in an orbit

19
Q

What is the perihelion?

A

The closest point to the sun in an orbit

20
Q

What is meant by the level of eccentricity when looking at somethings orbit?

A

It is a measure of how elongated a circle is
-low eccentricity= circle
-high eccentricity= elliptical

21
Q

What is Keplers second law?

A

A line segment joining a planet and the sun sweeps out equal areas during equal intervals of time.
(As planets move around the sun the speed is not constant, planet closer to sun=move faster)(time interval=same)

22
Q

What is Keplers third law?

A

The square of the orbital period is directly proportional to the cube of its average distance form the sun

23
Q

What is one astronomical unit?

A

The mean distance between the earth and the sun

24
Q

How do i work out the orbital speed of a planet?

A

-Equate the centripetal force of the planet to the gravitational force on the planet and rearrange for v.
-2pi divided by time = velocity (2pi=circumference=distance)

25
Q

How do you derive the orbital speed for a stable distance r from the centre of mass of the earth?

A

By equating the centripetal force and the gravitational force
-giving you v=square root (GM/r)

Mass of the satellite is not a factor in this equation

26
Q

Even if the mass of a satellite varies will all satellites at a given height be travelling at the same speed?(in a given orbit)

A

Yes it will be the same (the mass is not taken into account)

27
Q

What do satellites with a polar orbit circle?

A

The poles

28
Q

What are polar satellites used for?

A

-covers all parts of the globe (as earth spins on its axis)
-used for mapping
-reconnaissance (military use)

29
Q

What 3 points must a satellite follow to be a geostationary satellite?

A

-be in orbit above the earths equator
-rotate in the same direction as the earths rotation
-have an orbital period of 24 hours

30
Q

What is the gravitational potential at a point in a gravitational field defined as?

A

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

31
Q

What does infinity refer to in terms of gravitational potential?

A

To the distance so far from the object producing the gravitational field that the gravitational field strength is zero

32
Q

is gravitational potential a scalar or vector quantity?

A

It is a scalar quantity

33
Q

What are all values of gravitational potential?

A

Negative

34
Q

Why are all values of gravitational potential negative?

A

they are all negative because:
-all masses attract each other
-takes energy(external work must be done) to move objects apart

35
Q

Where is gravitational potential maximum?

A

At infinity
-value=0Jkg^-1

36
Q

What is the gravitational potential when r is equal to infinity?

A

0JKg^-1

37
Q

What does moving towards a point mass do to the gravitational potential?

A

Results in a decrease in gravitational potential

38
Q

What does moving away from a point mass do to the gravitational potential?

A

Results in an increase in gravitational potential

39
Q

What is the definition of gravitational potential energy of any object of mass m within a gravitational field?

A

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

40
Q

How do you derive the equation for the escape velocity?

A

Equate the kinetic energy to the gravitational potential energy then rearrange for v

41
Q

What must be the supplied energy of an object in order to escape a gravitational field?

A

The energy must be equal to the gain in gravitational potential energy needed to lift it out of the field.

Physics alevel ocr a (26 decks)

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