Gravity (Pres V) Flashcards
(24 cards)
Which two forces apply to earthbound objects? And why?
Gravitational force (because of mass) Centrifugal force (because of rotation)
The sum of gravitational force and centrifugal force is known as what? And what is the equation?
Gravity force. F (prime) = F + f
What is the equation for gravitation?
g = (G*M)/r^2
G = 6.67 x 10^6 M = mass of earth r = radius of earth
What is the equation for centrifugal force?
f = w^2*p
w = angular velocity of earth
Is gravitational or centrifugal force greater?
Gravitational is much stronger
Where are G and C max and min?
G is max at poles (closer to center of earth) and min at equator
C is max at equator (most rotation) and zero at poles
What do the forces on earth cause it to be like?
Ellipsoidal and rotating
What is the magnitude of gravity measured in?
Gals (after Galileo)
1 Gal = 1 cm/s^2
g = 9.8 m/s^2 = 980 cm/s^2 = 980 Gals
How much does gravity change over the earth?
5 Gals
5/1000 = 0.05
200lbs* 0.005 = 10x1^-1 = 1lb lighter at poles
How is gravity measured? (two ways)
Absolute: refer to the standards of length and time
Relative: (measured much faster & easier) determination of gravity differences
Name some examples of the absolute method.
Pendulum Method
Free Fall Devices
Superconducting Gravimeter
Pendulum method
Measure both length and time of a freely swinging pendulum
T=2pisqrt(1/g)
Because friction, not that precise
Accuracy = dev appx 100 uGal
Free Fall Devices
Based on the equation of motion of a freely falling body
Dropping sapphires few thousand times of 48 hours…obtains 1 ugal accuracy
d=do+vot+1/2g*t^2
One of most accurate devices we have
Superconducting Gravimeter
Extremely high precision
~10^-10 Gal
What is the equation for gravity measurements?
mg=kl
What do the variables mean in the equation for gravity measurements?
k = stiffness l = length m = mass g = gravity
How does the LaCoste & Rombery Type Gravimeter work? (Relative)
It contains a zero length spring and measures gravity as length of spring changes
*Not measuring time
Inertial Survey Systems
Observe accelerometers movement up and down
Not too accurate
Why must gravimeters be heavily packaged
They are very sensitive devices
What type of surveys are gravity surveys similar too?
Leveling surveys
Profile method
Star method
Step method
What is the Gravity Reference System needed for?
It’s a global system needed to refer local & regional gravity networks to a common standard
What are some gravity surveys used for?
To find underground resources such as oil, diamonds, etc
Who collects gravity survey data in the US?
NGS, USGS, and some private companies
What is the gravity anomaly?
It’s the difference in gravity with respect to a reference value.
