Gravitation

Question 1

Newton’s law of Gravitation

Answer 1

Every particle attracts every other particle 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

Gravitational field strength

Answer 2

The gravitational field strength at a point is defined as the gravitational field strength per unit mass acting on a mass placed at that point.

Question 3

Gravitational potential energy

Answer 3

The GPE of a mass at a point in the gravitational field is the the work down by an external force in bringing the mass from infinity to that point without a change in kinetic energy.

Question 4

Gravitational potential

Answer 4

The gravitational potential at a point in a gravitational field is the work done per unit mass by an external force in bringing a mass from infinity to that point without a change in kinetic energy

Question 5

Weightless vs apparently weightless

Answer 5

• truly when no Fg acting on it

- object apparently weightless when it exerts no contact force on its support

Question 6

Rs between Fg and U; g and potential

Answer 6

Fg = -dU/dr
g = - d potential/ dr

Question 6

Characteristics of Geostationary satellites

Answer 6

• period is 24 h
• direction of rotation from west to east
• plane of rotation is equator

Question 7

Advantages and disadvantages of geostationary satellites

Answer 7

Advantages:

• communication: require no tracking to receive signals as they remain stationary at the same point above earth
• meteorology and imaging: can view almost the whole earth disk as they are positioned at high altitudes

Disadvantages:

• spatial resolution of images low as they are positioned at high altitudes
• cannot see N and S poles and limited use for latitudes greater than 60-70 deg N or S as they positioned along equator

Question 8

Neutral point

Answer 8

It is the point where the g field strength due to Earth is equal in magnitude and opposite to gravitational field strength due to moon

• minimum speed for object to reach earth from surface of moon is lower than escape V
• GPE at earth surface less than 0
• change in PE of stone from moon to E less than to go to infinity
• by COE less KE requires at the start and hence less minimum v needed

Question 9

Why is U negative

Answer 9

Gravitational force is attractive in nature.

Positive work must be done by an external force to increase the separation between 2 objects, hence GPE increases as distance between 2 objects increase.

Since GPE is 0 at infinity, masses must have GPE lower than 0 when they are finite distance apart.