Chapter 7: Gravitational Field

Question 1

State Newton’s Law of Gravitation

Answer 1

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 2

What is a gravitational field?

Answer 2

It is a region of space where a mass experiences a force due to the presence of another mass

Question 3

Define gravitational field strength

Answer 3

It is the gravitational force per unit mass acting on a test mass at that point

Question 4

What is a geostationary orbit?

Answer 4

It is a circular orbit around the Earth that appears stationary to observers on Earth’s surface

Question 5

What are the features of geostationary satellites?

Answer 5

1. Located at the same height above Earth’s surface, directly above the equator
2. All geostationary satellites have the same linear speed v

Question 6

What is a polar orbit?

Answer 6

Satellites orbit about a plane containing the North and South poles, and the centre of the Earth

Question 7

Define gravitational potential at a point

Answer 7

It is the work done per unit mass needed by an external agent to bring a test mass from infinity to that point (without a change in KE)

Question 8

Why is gravitational potential always negative

Answer 8

• The gravitational potential at infinity is zero
• Gravitational pull is attractive in nature
• The external force must be equal in magnitude and opposite in direction to gravitational pull such that there is not change in KE
• Since the direction of external force is opposite to the displacement of the mass (positive), gravitational potential is negative

Question 9

Define gravitational potential energy

Answer 9

It is the work done by an external agent in bringing a test mass from infinity to that point (without a change in KE)

Question 10

Graphically, what is gravitational force equal to?

Answer 10

It is equal to the negative gravitational potential energy gradient (-dU/dt)

Question 11

Graphically, what is gravitational field strength equal to?

Answer 11

It is equal to the negative gravitational potential gradient

Question 12

Define escape speed

Answer 12

It is the speed required for a body to escape a gravitational field

v = sqrt(2GM/R) = sqrt(2gR)

Question 13

How is orbital period related to orbital radius?

Answer 13

T^2