Gravity (Pres V)

Question 1

Which two forces apply to earthbound objects? And why?

Answer 1

Gravitational force (because of mass)
Centrifugal force (because of rotation)

Question 2

The sum of gravitational force and centrifugal force is known as what? And what is the equation?

Answer 2

Gravity force. F (prime) = F + f

Question 3

What is the equation for gravitation?

Answer 3

g = (G*M)/r^2

G = 6.67 x 10^6
M = mass of earth
r = radius of earth

Question 4

What is the equation for centrifugal force?

Answer 4

f = w^2*p

w = angular velocity of earth

Question 5

Is gravitational or centrifugal force greater?

Answer 5

Gravitational is much stronger

Question 6

Where are G and C max and min?

Answer 6

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

Question 7

What do the forces on earth cause it to be like?

Answer 7

Ellipsoidal and rotating

Question 8

What is the magnitude of gravity measured in?

Answer 8

Gals (after Galileo)
1 Gal = 1 cm/s^2
g = 9.8 m/s^2 = 980 cm/s^2 = 980 Gals

Question 9

How much does gravity change over the earth?

Answer 9

5 Gals

5/1000 = 0.05
200lbs* 0.005 = 10x1^-1 = 1lb lighter at poles

Question 10

How is gravity measured? (two ways)

Answer 10

Absolute: refer to the standards of length and time
Relative: (measured much faster & easier) determination of gravity differences

Question 11

Name some examples of the absolute method.

Answer 11

Pendulum Method
Free Fall Devices
Superconducting Gravimeter

Question 12

Pendulum method

Answer 12

Measure both length and time of a freely swinging pendulum
T=2pisqrt(1/g)
Because friction, not that precise
Accuracy = dev appx 100 uGal

Question 13

Free Fall Devices

Answer 13

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

Question 14

Superconducting Gravimeter

Answer 14

Extremely high precision

~10^-10 Gal

Question 15

What is the equation for gravity measurements?

Answer 15

mg=kl

Question 16

What do the variables mean in the equation for gravity measurements?

Answer 16

k = stiffness
l = length
m = mass
g = gravity

Question 17

How does the LaCoste & Rombery Type Gravimeter work? (Relative)

Answer 17

It contains a zero length spring and measures gravity as length of spring changes

*Not measuring time

Question 18

Inertial Survey Systems

Answer 18

Observe accelerometers movement up and down

Not too accurate

Question 19

Why must gravimeters be heavily packaged

Answer 19

They are very sensitive devices

Question 20

What type of surveys are gravity surveys similar too?

Answer 20

Leveling surveys
Profile method
Star method
Step method

Question 21

What is the Gravity Reference System needed for?

Answer 21

It’s a global system needed to refer local & regional gravity networks to a common standard

Question 22

What are some gravity surveys used for?

Answer 22

To find underground resources such as oil, diamonds, etc

Question 23

Who collects gravity survey data in the US?

Answer 23

NGS, USGS, and some private companies

Question 24

What is the gravity anomaly?

Answer 24

It’s the difference in gravity with respect to a reference value.