Reading: / Sentence Completion / Part 1

A. Without forces of gravitation, Earth and other planets would be unable to stay in their orbits around the Sun, the Moon would be unable to orbit the Earth, tidal waves would not occur and the rising of hot air or water convection would be impossible. Gravitation is a phenomenon which allows objects to attract other matter; the physics behind it have been explained in The Theory of Relativity and Newton’s Law of Universal Gravitation; though attempts to explain gravity hail back to ancient times. In 4th Century B.C. the Greek philosopher Aristotle developed the hypothesis that all objects were drawn into their correct position by crystalline spheres and that a physical mass would fall towards the earth in direct proportion to its weight. 

B. In the late 16th century Galileo deduced that while gravitation propels all objects to the ground at the same rate, air resistance resulted in heavier objects appearing to fall more quickly; his theories contradicting earlier belief systems put in place by Aristotle and others; so paving the way for formulation of the modern theories of today. Though the two terms are now used interchangeably in layman use, strictly by scientific definition, there are distinct differences between ‘gravitation’ and ‘gravity’. The first relates to the influence exerted by different objects which allow them to attract other objects, whereas ‘gravity’ refers specifically to the force possessed by such objects which facilitates gravitation. Certain scientific theories hold that gravitation may be initiated by a combination of factors and not simply the existence of gravity alone; though doubts have been raised regarding some of these theories. 

C. Gravity is directly proportional to mass; a smaller object possessing less gravity. To illustrate, the Moon is a quarter of the Earth’s size and possesses only 1/6 of its gravity. The mass of the Earth itself is not spread out proportionally, being flatter at the poles than the equator as a result of its rotation; gravity and gravitational pull in different locations throughout the world also vary. In the 1960s, as a result of research into the worldwide gravity fields, it was discovered that inexplicably areas around and including the Hudson Bay area of Canada appeared to possess significantly lower levels of gravity than other parts of the globe; the reasons for this dissimilarity have since been extensively investigated resulting in two explanations.

D. The original theory presented attributed this anomaly to activity which occurs 100-200 kilometres below the Earth’s surface within the layer known as the ‘mantle’. The mantle is comprised of hot molten rock known as magma which flows under the earth’s surface causing convection currents. These convection currents can result in the lowering of the continental plates which make up the Earth’s surface, as a result when this occurs, the mass in that area and its gravity is also reduced. Research findings indicated that such activity had occurred in the Hudson Bay region.

E. More recently a second conjecture suggested that, in fact, lower levels of gravity in the area are a result of occurrences during the Ice Age. The Laurentide Icesheet, which covered most of Canada and the northern tip of the USA until it melted 10,000 years ago, is thought to have been 3.2 kms thick in most parts and 3.7 kms thick over two areas of Hudson Bay. The sheer weight of the ice layer weighed down the surface of the earth below, leaving a deep indentation once it had melted, having caused the area around Hudson Bay to become thinner as the earth’s surface was pushed to the edges of the icesheet.



Questions 1-4

Complete the sentences below with words from the box below.

Write the correct letter A-J in boxes 1-4 on your answer sheet.

1. The impact of _____________ on objects falling to the ground was not considered by Aristotle.

2. Investigations of _________________ first led to the discovery of the unusual levels in Hudson Bay.

3. The earth's surface has been observed to sink as a direct result of ____________

4. The largest proportion of the Laurentide icesheet was ____________ in depth.

a. crystalline spheres

b. mass

c. 3.2 kms

d. continental plates

d. 3.7kms

e. gravity fields

                             f. warming

g. 3.5 kms

h. mantle layers

i. convection currents

     j. air resistance