Text 3. Gravity

Gravity is the force that pulls all the objects in the universe toward each other. On the Earth gravity pulls all the objects “downward’

toward the center of the planet. According to Sir Isaac Newton’s Universal Law of Gravitation, the gravitational attraction between two bodies is stronger when the masses of the objects are greater and closer together. This rule applies to the Earth’s gravitational field as well. Because the Earth rotates and its mass and density vary at different locations on the planet, gravity is also varied.

One reason why geodesists measure variations in the Earth’s gravity is because gravity plays a major role in determining sea level.

Geodesists calculate the elevation of locations on the Earth’s surface based on the mean sea level.

So knowing how gravity changes sea level helps geodesists make more accurate measurements.

In areas of the planet where gravitation is stronger, the mean sea level will be higher. In areas where the Earth’s accurate measurements are weaker, the mean sea level will be lower.

To measure the Earth’s gravity field, geodesists use instruments in space and on land. Space satellites gather data of gravitational changes as they pass over points on the Earth’s surface. On land devices called gravimeters measure the Earth’s gravitational pull on a suspended mass. With this data geodesists can create detailed maps of gravitational fields and adjust elevations on existing maps. Gravity principally affects the vertical datum because it changes the elevation of the land surface.

Exercise 16. Look through the words and try to remember their meaning:

gravity —сила тяжіння

gravitational attraction —гравітаційне притягання

gravitational field —гравітаційне поле

accurate measurements —точні вимірювання

mean sea level —середній рівень моря

density —щільність

to rotate —обертатися

to pass over —проходити повз

to create detailed maps —створювати детальні карти

to adjust elevations —вивіряти, встановлювати підняття

to affect —впливати

to calculate —обчислювати, розраховувати

to determine —визначати, встановлювати

Exercise 17. Answer the following questions:

1. What is gravity?

2. What does Sir Isaac Newton’s Universal Law of Gravitation tell us about?

3. Why do geodesists measure variations in the Earth’s gravity?

4. What do geodesists use to measure the Earth’s gravity field?

5. Where and how do satellites gather data of gravitational changes?

6. How can geodesists create detailed maps?

7. How does gravity principally affect the vertical datum?

Exercise 18. Translate the following word combinations:

gravitational attraction, gravitational field, mean sea level, gravity changes, accurate measurements, space satellites, land devices, suspended mass, elevation of the land surface.

Exercise 19. Complete the following chart:

Verb	Noun	Adjective
to attract
to rotate
to vary		variable
to create
to calculate	calculation
to mean
to base
to change
to pass
to affect

Exercise 20. Look through the following verb forms and define the tense, write the infinitive of the verb:

applies, rotates, make, called, pull, can create, adjust, were detailed, affects, will be changed, is varied, has passed, have being made.

Exercise 21. Put verbs in the brackets in the correct form:

1. On Earth gravity ________(to pull) all the objects “downward’ toward the center of the planet.

2. This rule _________(to apply) to the Earth’s gravitational field as well.

3. Geodesists measure variations in the Earth’s gravity _____(to be) because gravity_______(to play) a major role in determining sea level.

4. So knowing how gravity changes sea level _____ (to help) geodesists make more accurate measurements.

5. Space satellites ______ (to gather) data of gravitational changes as they ________ (to pass) over points on the Earth’s surface.

6. Gravity principally _______ (to affect) the vertical datum because it changes the elevation of the land surface.

Exercise 22. Change the following sentences into Passive:

1. On the Earth gravity pulls all the objects “downward’ toward the center of the planet.

2. Geodesists calculate the elevation of locations on the Earth’s surface based on the mean sea level.

3. One reason why geodesists measure variations in the Earth’s gravity is because gravity plays a major role in determining sea level.

4. Space satellites gather data on gravitational changes as they pass over points on the Earth’s surface.

5. With these data geodesists can create detailed maps of gravitational fields and adjust elevations on existing maps.

Exercise 1. Read and translate the text.