Chapter 18 gravitational fields

Question 1

What are gravitational foibles due to?

Answer 1

Mass

Question 2

How is the mass of a spherical object modelled?

Answer 2

As a point at the centre of mass

Question 3

What do all objects with mass create?

Answer 3

A gravitational field

Question 4

Where do gravitational fields extend to?

Answer 4

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

Question 5

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

Answer 5

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)

Question 6

What is the gravitational field strength equal to on earth?

Answer 6

9.81ms^-2 (free fall)

Question 7

Is gravitational force always attractive?

Answer 7

Yes

Question 8

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

Answer 8

The field lines are closer together

Question 9

What do field lines form around a spherical mass?

Answer 9

A radial field

Question 10

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

Answer 10

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

Question 11

Does the gravitational field strength change in a uniform field?

Answer 11

No

Question 12

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

Answer 12

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)

Question 13

What does Newtons law of gravitation state?

Answer 13

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)

Question 14

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

Answer 14

It shows that the gravitational force is an attractive force.

Question 15

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

Answer 15

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

Question 16

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

Answer 16

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

Question 16

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

Answer 16

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)

Question 17

What is Keplers first law?

Answer 17

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)

Question 18

What is the aphelion?

Answer 18

The furthest point from the sun in an orbit

Question 19

What is the perihelion?

Answer 19

The closest point to the sun in an orbit

Question 20

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

Answer 20

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

Question 21

What is Keplers second law?

Answer 21

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)

Question 22

What is Keplers third law?

Answer 22

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

Question 23

What is one astronomical unit?

Answer 23

The mean distance between the earth and the sun

Question 24

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

Answer 24

-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)

Question 25

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

Answer 25

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

Question 26

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)

Answer 26

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

Question 27

What do satellites with a polar orbit circle?

Answer 27

The poles

Question 28

What are polar satellites used for?

Answer 28

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

Question 29

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

Answer 29

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

Question 30

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

Answer 30

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

Question 31

What does infinity refer to in terms of gravitational potential?

Answer 31

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

Question 32

is gravitational potential a scalar or vector quantity?

Answer 32

It is a scalar quantity

Question 33

What are all values of gravitational potential?

Answer 33

Negative

Question 34

Why are all values of gravitational potential negative?

Answer 34

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

Question 35

Where is gravitational potential maximum?

Answer 35

At infinity -value=0Jkg^-1

Question 36

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

Answer 36

0JKg^-1

Question 37

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

Answer 37

Results in a decrease in gravitational potential

Question 38

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

Answer 38

Results in an increase in gravitational potential

Question 39

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

Answer 39

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

Question 40

How do you derive the equation for the escape velocity?

Answer 40

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

Question 41

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

Answer 41

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