7: Gfield

Question 1

Define Newton’s Law of Gravitation

Answer 1

NL of Gravitation states that two point masses attract each other with a force that is proportional to the product of their masses and inversely proportional to the square of their separation between them.

Question 2

Define G field

Answer 2

Gfield is a region of space in which a mass experiences gravitational force

Question 3

Define Gravitational Field Strength

Answer 3

GFS at a point in the gravitational field refers to the force per unit mass exerted on a small test mass placed at that point.

Question 4

Define GPE

Answer 4

GPE of a small test mass at a point in the gravitational field refers to the work done by an external agent in bringing the mass from infinity to that point.

Question 5

Define Gravitational potential

Answer 5

Gravitational Potential of a small test mass at a point in a gravitational field is the work done per unit mass in bringing the mass from infinity to that point.

Question 6

Explain why GPE/GP is negative

Answer 6

1. GPE at infinity is zero
2. External force by agent is must be opposite to the attractive gravitational force to ensure that net force is zero so that there is no change in energy to the system
3. As the force brings the mass from infinity in the direction of gravitational force, opposite to direction of external force,
4. As work done = force x displacement, thus negative work is done by the force (energy transferred to the object)

Question 7

In your mind,
1. Sketch g-r graph
2. Sketch the corresponding GP-r graph

Answer 7

g = negative of gradient of GP-r graph

Question 8

State 3 advantages of geostationary orbit

Answer 8

1. Continuous surveillance of one area
2. Easy communication - as it is easy to locate
3. Can receive and transmit signals over large area - due to its high altitude

Question 9

State 3 disadvantages of geostationary orbit

Answer 9

Reason: Altitude of such satellites in orbit is very high (large radius needed for cm)
1. Significant loss of signal strength
2. Poorer resolution in imaging
3. Time lag in communications

Question 10

Why does orbital decay occur?

Answer 10

Due to work done against atmosphere friction