Chapter 4 - Gravity Flashcards
What direction does gravity act?
It acts towards the centre of mass.
3 rules for drawing a field?
- use lines
- arrows for direction
- lines never touch or cross
3 rules for gravitational field lines on sphere?
- Enter at right angle to surface of Earth
- fade out towards centre of Earth
- Start at infinity and end on matter
Gravitational force - distance law?
The gravitational force between two objects is inversely proportional to the square of the distance between them.
F = k(1/d^2)
What is gravitational field strength (GFS)?
The force per unit mass at any point (N/kg).
GFS equation?
GFS = force/mass (N/kg)
What is the difference between GFS and g?
GFS - tells you about the gravitational pull of one body on another
g - tell you about the acceleration of a mass under a force
When are point masses appropriate?
When the distance between objects is much more than the size of the objects.
What is Newtons law of gravity?
The attractive force between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them.
Newtons equation for gravity?
F = -GMm/r^2
What is a sidereal day?
The time taken for the Earth to rotate on its axis 360degrees (23hrs 56mins).
What is a solar day?
The time taken for the sun to be in the same point in the sky (24hours).
Equation for g used to find GFS of a point mass?
g= -GM/r^2
Equation joining CPF and F?
F = -GMm/r^2 CPF = mv^2/r
mv^2/r = GMm/r^2
v^2r/G = M
Equation joining both Fs?
F = -GMm/r^2 F = mg
g =GM/r^2
Why do astronauts appear weightless?
Because their gravitational force (weight) is being used to keep them in orbit (it provides CF and acceleration).
Keplers 3rd law?
T^2 is proportional to r^3
Keplers equation and how to get there?
T^2= (4π^2/GM)r^3
v=2πr/T
v^2 = 4π^2/T^2
Also v^2 = GM/r
Equating: 4π^2/T^2 = GM/r
Rearrange to get final equation
Define satellite?
A body that orbits something else.
4 rules for geostationary satellites?
- Orbital period = 24hours
- Spins same way as Earth (W to E)
- Centred on centre of Earth
- In line with equator
Define geostationary satellite?
.
Uses of geostationary satellites?
Weather forecasting
Radio
Tv
Problems with geostationary satellites?
-Very expensive - costs lots to get it into orbit since large gains in KE and GPE must be made.
Define a field?
A region in which an object may experience a force.
