Gravitational Fields

Question 1

What is a gravitational field?

Answer 1

A region of space in which an object with mass will experience a gravitational force.

Question 2

What is gravitational field strength?

Answer 2

The force per unit mass exerted on test mass at that point in the field.

Question 3

What are the units of gravitational field strength?

Answer 3

N kg⁻¹

Question 4

What does the direction of a gravitational field line show?

Answer 4

The direction of force on a test mass at that point in the field.

Question 5

What is represented by the spacing of field lines in a field line diagram?

Answer 5

Field strength.

Question 6

What is a radial gravitational field?

Answer 6

A gravitational field produced by a spherical (or point) mass in which the field lines all point towards the centre of mass of the object causing the field.

Question 7

What is a uniform gravitational field?

Answer 7

A region in which the gravitational field strength is constant.

Question 8

How is a uniform gravitational field shown in a field line diagram?

Answer 8

Parallel, equidistant field lines

Question 9

What is a point mass?

Answer 9

A idealised mass with zero size. The field outside a spherical mass can be modelled as the field around a point mass.

Question 10

State Newton’s Law of Gravitation.

Answer 10

The gravitational attraction between two objects is proportional to the product of their masses and inversely proportional to the sqaure of the distance between their centres of mass

Question 11

State Kepler’s 1st Law.

Answer 11

A planet’s orbit is an ellipse with the Sun at one of the two foci

Question 12

What is an aphelion?

Answer 12

The point in an orbit when an object is furthest from the sun and travelling slowest.

Question 13

What is a perihelion?

Answer 13

The point in an orbit when an object is closest to the sun and travelling fastest.

Question 14

State Kepler’s 2nd Law.

Answer 14

A line segment joining the planet to the Sun sweeps out equal areas in equal time intervals.

Question 15

State Kepler’s 3rd Law.

Answer 15

For planets in the solar system the square of the orbital period is proportional to the cube of the average orbital radius.

Question 16

How does graviational field strength vary with distance from a point mass?

Answer 16

It follows an inverse square law.

Question 17

What is geostationary satellite?

Answer 17

A satellite with an orbital period of 24 hours which orbits above the equator in the same direction as the Earth’s rotation so its apparent position in the sky remains unchanged.

Question 18

What are the disadvantages of geostationary satellite

Answer 18

1. A geostationary orbit is quite far (42000 km form centre of the earth), so it might have poor resolution for imaging and some delays in communication
2. Always point towards a specific point on earth

Question 19

What is the gravitational potential of a point?

Answer 19

The work done per unit mass to move an object from an infinite distance away to that point in the field.

Question 20

How does graviational potential vary with distance from a point mass?

Answer 20

It is inversely proportional to distance.

Question 21

What is escape velocity?

Answer 21

The velocity at which the magnitude of an object’s kinetic energy is larger than the magnitude of its gravitational potential energy.

Question 22

What is gravitational potential energy?

Answer 22

The work done to move an object from infinity to a point in the gravitational field.

Question 23

What is the equation fro gravitational potential energy in a radial field?

Answer 23

U=-(GMm)/r

Question 24

What is the total mechanical energy of an orbiting object?

Answer 24

E=GPE +KE =-(GMm)/r+(GMm)/2r=-(GMm)/2r

Question 25

What is the kinetic energy of an orbiting object?

Answer 25

KE=(GMm)/2r