# 5.4 Gravitational Fields Flashcards Preview

## Physics A Level > 5.4 Gravitational Fields > Flashcards

Flashcards in 5.4 Gravitational Fields Deck (29)
1
Q

What causes gravitational fields?

A

Any object with mass

2
Q

How do gravitational field lines represent gravitational fields? (3)

A

At right angles to object
Arrows show direction of force (always attractive)
Separation of lines shows strength of field

3
Q

Define gravitational field strength

A

The gravitational force exerted per unit mass at that point in the gravitational field

4
Q

How is gravitational field strength and distance related?

A

Gravitational field strength is inversely proportional to the distance from the centre of mass of the object

5
Q

What is assumed about the Earth’s gravitational field strength?

A

It is constant close to the surface of the Earth as the field is considered to be uniform

6
Q

What does Newton’s law of gravitation assume? (2)

A

Point masses

Mass is concentrated at centre of mass

7
Q

A

Gravity is always an attractive force

8
Q

What is Kepler’s first law of planetary motion?

A

The orbit of a planet is an ellipse with the sun at one of the two foci

9
Q

How is a gravitational field shown to be uniform?

A

Equidistant, parallel field lines

10
Q

State Newton’s law of gravitation

A

The force between two point masses is directly proportional to the product of the masses and inversely proportional to the square of their separation

11
Q

What is eccentricity?

A

A measure of how elongated a circle is

12
Q

What is Kepler’s second law of planetary motion?

A

A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time

13
Q

What is Kepler’s third law of planetary motion?

A

The square of the orbital period of a planet is directly proportional to the cube of its average distance from the Sun

14
Q

How is Kepler’s third law derived? (3)

A

Model the planetary orbit as a circle
Centripetal force on planet = gravitational force on planet
Determine speed by dividing circumference by orbital period

15
Q

What is a geostationary satellite?

A

A satellite that stays above the same area of Earth

16
Q

What are the conditions for a geostationary satellite? (4)

A

36000km from Earth’s surface
Orbit above the equator
Time period of 24 hours
Orbit in the same direction as Earth’s rotation

17
Q

What are polar orbits used for? (2)

A

Mapping

Reconnaissance

18
Q

What are Low Earth Orbits used for? (2)

A

Photography

Weather

19
Q

What are Medium Earth Orbits used for?

A

GPS

20
Q

What are geostationary orbits used for? (2)

A

Communications

TV

21
Q

What are High Earth Orbits used for?

A

Research

22
Q

Define gravitational potential

A

The work done per unit mass to move an object to a point in a gravitational field from infinity

23
Q

What does infinity refer to in the definition of gravitational potential?

A

A distance so far from the gravitational field that its field strength is zero

24
Q

What is true about gravitational potential at infinity?

A

It is zero

25
Q

Why are all values of gravitational potential negative? (2)

A

All masses attract each other

Work must be done to separate objects gravitational potential is zero at infinity

26
Q

What does a graph of gravitational potential against distance look like? (3)

A

Most negative value at surface of object (graph starts here)
Inversely proportional relationship
All values of gravitational potential are negative

27
Q

How does gravitational potential change when masses move? (2)

A

Moving towards a point mass decreases gravitational potential (more negative)
Moving away from a point mass increases gravitational potential (less negative)

28
Q

What does the area under a force-distance graph for a point mass represent?

A

Work done

29
Q

What is escape velocity? (2)

A

The minimum velocity required to escape the gravitational field of a mass
When loss in KE = gain in GPE