Newton’s law of Gravitation
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
Gravitational field strength
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.
Gravitational potential energy
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.
Gravitational potential
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
Weightless vs apparently weightless
- truly when no Fg acting on it
- object apparently weightless when it exerts no contact force on its support
Rs between Fg and U; g and potential
Fg = -dU/dr g = - d potential/ dr
Characteristics of Geostationary satellites
- period is 24 h
- direction of rotation from west to east
- plane of rotation is equator
Advantages and disadvantages of geostationary satellites
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
Neutral point
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
Why is U negative
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.
