Gravitational Fields

Question 1

What is gravity?

Answer 1

Gravity is the universal attractive force which acts between all matter

Question 2

What is a radial field?

Answer 2

A field in which field lines radiate towards a central mass.

The strength decreases with distance according to the inverse square law.

Question 3

What do field lines show?

Answer 3

• The direction of the field (towards the mass)
• The strength of the field (closer lines = stronger field)

Question 4

What is gravitational potential?

Answer 4

the potential energy per unit mass at a point in a gravitational field.

Question 5

Why is gravitational potential negative?

Answer 5

• Zero potential is defined at infinity.
• Work must be done to move a mass from inside the field to infinity.
• Therefore, potential energy within the field is negative.

Question 6

What is gravitational potential difference

Answer 6

The difference in gravitational potential between two points in a field.

Question 7

What is an equipotential surface?

Answer 7

A surface where every point has the same gravitational potential.

Question 8

How much work is done moving along an equipotential?

Answer 8

No work is done.

Question 9

Compare the PE and KE of a lower orbit to a higher one.

Answer 9

Lower orbit (smaller r):
* More negative potential energy
* Greater kinetic energy
* Higher speed

Higher orbit:
* Less negative potential energy
* Lower kinetic energy
* Slower speed

Question 10

What is escape velocity?

Answer 10

Minimum speed needed to escape a gravitational field without further propulsion.

Question 11

Kepler’s Third Law

Answer 11

for objects in circular orbit around the same central mass, the square of the orbital period is proportional to the cube of the orbital radius.

Question 12

Geostationary Satellite

Answer 12

• Orbits the Earth
• Has an orbital period of 24 hours
• Moves in the same direction as Earth’s rotation
• Lies in the equatorial plane
• Remains fixed above one point on Earth’s surface

Question 13

When does Kepler’s Third Law apply?

Answer 13

• Circular orbits
• Objects orbiting the same central mass

Question 14

What does Kepler’s Third Law show about outer planets?

Answer 14

Planets further from the Sun have much longer orbital periods.

Question 15

What is a synchronous orbit?

Answer 15

An orbit where the satellite’s orbital period equals the rotation period of the planet.

Question 16

Are all synchronous orbits geostationary?

Answer 16

No.