Chapter 18: Gravitational Fields

Question 1

What is gravity?

Answer 1

Gravity is the universal attractive force which acts between all matter

Question 2

What is G?

Answer 2

The universal gravitational constant
6.67 x 10^-11

Question 3

What can field lines tell you about a field?

Answer 3

the direction of the field and the strength of the field depending on the density of the field lines

Question 4

What is g?

Answer 4

• force per unit area in a uniform gravitational field
• in a radial field, magnitude of g is the proportionality constant at that point between force and mass

Question 5

What is Kepler’s First law?

Answer 5

The orbit of a planet is an ellipse with the sun at one focus, the eccentricity of the ellipse is very low, so the motion can be modelled as circular

Question 6

What is Newton’s law of Gravitation?

Answer 6

Two point masses attract each other with a force that is directly proportional to the product of their masses, and inversely proportional to the square of the distance between them

Question 7

What is Kepler’s second law?

Answer 7

a line segment joining a planet and the sun sweeps out equal areas during intervals of equal time. Speed of a planet is not constant - planet moves faster when closer to the sun

Question 8

What is Kepler’s third law?

Answer 8

the square of the orbital period T is proportional to the cube of the average distance r from the sun.

Question 9

What are satellites? What are they used for?

Answer 9

• Objects that orbit other, larger objects. - - Can be natural or artificial
• Uses: Communications, research, GPS

Question 10

What are geostationary satellites? What are they used for?

Answer 10

• Orbital period of exactly a day, appear stationary above Earth
• Orbit 36000 km above Equator
• Useful for communications and surveying

Question 11

What is gravitational potential?

Answer 11

energy per unit mass required to move an object from infinity to that point

Question 12

What is gravitational potential difference?

Answer 12

Difference in gravitational potential between two points in a gravitational field

Question 13

What is gravitational potential energy at a point in a field?

Answer 13

Work done to move an object from infinity to that point in the field

Question 14

What is escape velocity?

Answer 14

• minimum velocity an object requires in order to escape the gravitational field of an object when projected vertically from its surface.
• Formula derived from equating KE to GPE required to reach infinity

Question 15

How do gravitational fields arise?

Answer 15

They are around any object with a mass

Question 16

How is the gravitational field strength close to the Earth’s surface

Answer 16

uniform and acceleration is numerically equal

Question 17

Where does centripetal force on a planet come from?

Answer 17

gravitational force between planet and the Sun

Question 17

What quantity do you get from the area under a force-distance graph

Answer 17

work done