7.2 Gravitational Fields

Question 1

what is Newton’s law?

Answer 1

Gravity acts on any objects which have mass and is always attractive.

Question 2

what does Newton’s law of gravitation show?

Answer 2

the magnitude of the gravitational force between two
masses is directly proportional to the product of the masses, and is inversely proportional to the square of the distance between them.

Question 3

What are the two types of gravitational field?

Answer 3

a uniform field or radial field

Question 4

What are the arrows used for in both diagrams of gravitational fields?

Answer 4

The arrows on the field lines show the direction that a force acts on a mass in a force field

Question 5

What is a uniform field?

Answer 5

exerts the same gravitational force on a mass everywhere in the field, as shown by the parallel and
equally spaced field lines.

Question 6

What is a radial field?

Answer 6

the force exerted depends on the position of the
object in the field - as an object moves further away from the centre, the
magnitude of force would decrease because the distance between field lines decreases

Question 6

Gravitational field strength (g)

Answer 6

the force per unit mass exerted by a gravitational field on an
object

Question 7

What equation do you use for only radial fields

Answer 7

g = GM/r^2

Question 8

Define gravitational potential at a point

Answer 8

is the work done per unit mass when moving an object
from infinity to that point.

Question 9

When is Gravitational potential zero?

Answer 9

At infinity , as an object moves from
infinity to a point, energy is released as the gravitational potential energy is reduced, therefore
gravitational potential is always negative.

Question 10

Gravitational potential (V) equation

Answer 10

V = -GM/r

where r is the distance between the centres of the objects.

Question 11

What is gravitational potential difference( V Δ)?

Answer 11

is the energy needed to move a unit mass
between two points and therefore can be used to find the work done when moving an object in a
gravitational field.

Question 12

Work done equation

Answer 12

Work done = mΔV

Where m is the mass of the object moved

Question 13

What are equipotential surfaces?

Answer 13

• surfaces which are created through joining points of equal potential
  together, therefore the potential on an equipotential surface is constant everywhere.
• As these
  points all have equal potential, the gravitational potential difference is zero when moving along the
  surface, so no work is done when moving along an equipotential surface.

Question 13

What can you calculate from the Gravitational Potential graph?

Answer 13

Gravitational field strength (g)

Question 14

What is the gravitational field strength graph equation

Answer 14

g = −ΔV/Δr

Question 15

What happens if you plot a graph of gravitational field strength (g) against distance (r)?

Answer 15

you can find the gravitational
potential difference by finding the area under the
curve.

Question 16

What is a force field?

Answer 16

A region where an object will experience a non-contact force.

Question 17

Describe the gravitational field of the Earth.

Answer 17

• It is radial, so the field lines meet at the centre of the Earth like a spiderweb
• Close to the surface, the field can be considered almost uniform since the field lines are almost parallel and equally spaced

Question 18

What does Earths radial field look like?

Answer 18

Question 19

What is the equation of the time period of earths orbit?

Answer 19

T = 2πr/v

Question 20

What is escape velocity?

Answer 20

• The velocity at which an object’s kinetic energy is equal to minus its gravitational potential energy
• It is the minimum velocity at which an object must travel in order to escape a gravitational field

Question 21

Why is potential negative?

Answer 21

Have to do work against the field to move an object out of it.

Question 22

What is an object’s total energy when it travels at escape velocity?

Answer 22

• Zero

* Because the kinetic energy and GPE sum to 0 (since GPE is always negative)

Question 23

What is the equation for escape velocity?

Answer 23

v = √(2GM/r)

Question 24

Derive the equation for escape velocity.

Answer 24

• KE = 1/2mv²
• GPE = -GMm/r
• find v

Question 25

What force keeps an object undergoing circular motion in orbit?

Answer 25

Centripetal force

Question 26

Why is a satellite’s energy constant in circular orbit?

Answer 26

• Speed and distance above the Earth do not change
• So the kinetic energy and potential energy are constant
• So the total energy is always constant

Question 27

Why is a satellite’s energy constant in elliptical orbit?

Answer 27

• The satellite speeds up as it’s orbital radius decreases and slows down as orbital radius increases
• So kinetic energy increases as potential energy decreases (and vice versa)
• So the total energy remains constant

Question 28

What is a synchronous orbit?

Answer 28

Where the orbital period is the same as the rotational period of the orbited object.

Question 29

What is the time period of orbit of a geostationary satellite?

Answer 29

1 day

Question 30

What are geostationary satellites used for?

Answer 30

Sending TV and telephone signals.

Question 31

Compare the advantages of low orbit satellites and geostationary satellites.

Answer 31

Low orbit
* Cheaper to launch
* Require less powerful transmitters since they are close to Earth
Geostationary
* Do not require multiple satellites to achieve constant reception in one area