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Force, Gravity and Space-time Answers Worksheet 1 How do objects fall? Time to Think (p. 6) 1. People tended to accept the ideas or wisdom of the authority without question. 2. (a) No satisfactory result could be predicted. (b) The combined masses would fall at the same speed. 3. Release a heavy mass and a light mass of the same material from the same height and find whether the heavier mass will reach the ground faster than the lighter mass. Time to Think (p. 8) 4. The objective of the experiment is to find out whether the idea of Aristotle or Galileo is correct. 5. They watched which ball reached the ground first, and the position of the slower ball when the first ball reached the ground. 6. The result supported Galileo’s idea because both masses fell at the same speed as they reached the ground almost at the same time. 7. According to Aristotle, when a 100-pound ball reaches the ground, the 1-pound ball should have fallen for about 1/100 of the height. This means that if the total height is 100 cubits, the small ball should be about 99 cubits above the ground when the large ball reaches the ground. So the experimental error is about 99 cubits. But according Galileo, the experimental error is only 2 inches. 8. Galileo used inclined planes to slow down the fall of the balls. 9. The pulse of a person may vary from time to time and may be affected by factors such as emotion and physical activities of the person. 10. d = kt2 (or k = d/t2) where k is a constant. © Times Publishing (Hong Kong) Limited -1- Force, Gravity and Space-time Answers Worksheet 2 Understanding the motion of bodies on the Earth and in space Time to Think (p. 15) 1. When Newton saw an apple fall from a tree, he wondered whether the force which pulled the apple to the ground was the same force which held the Moon in its orbit around the Earth. 2. (a) A force called gravity acts on the stone by the Earth, causing it to fall faster and faster. (b) The Earth exerts an attractive force (the gravity) on the Moon, keeping the Moon in its orbit. 3. Both of the motions of a falling stone and the Moon are under the influence of gravity of the Earth. 4. This force exists between any two bodies with mass, no matter whether they are earthly or celestial bodies. 5. (a) The distance between the Equator and the centre of the Earth is greater than that between the North Pole and the centre of the Earth. The greater the distance between two bodies, the smaller is the gravity between them. (b) The mass of the Earth is about 100 times that of the Moon, and the radius of the Earth is about 4 times that of the Moon. Thus the gravity of the Earth is about 100/42 ≈ 6 times that of the Moon. Thus the gravity between the Moon and the person will be 1/6 of that between the Earth and the person. 6. He meant that his success had been based and built on the achievements of previous scientists. -2- © Times Publishing (Hong Kong) Limited Force, Gravity and Space-time Answers Worksheet 3 The greatest scientist in the 20th century Time to Think (p. 21) 1. Einstein worked in the field of theoretical physics, with mathematics as the instrument. 2. Nuclear power is the energy generated from the destruction of matter during nuclear reactions. This is not a kind of renewable energy. 3. People at that time firmly believed that light is a kind of wave. 4. Einstein used the kinetic theory to explain Brownian motion. To test his explanation, a microscope with very high power of magnification should be used to measure the rate of drifting of a suspended particle during Brownian motion. 5. One prediction is that when light from a distant star passes close to a large mass, such as the Sun, it will be deflected by a very small amount. This prediction is difficult to test because the Sun is so bright that a slight deflection of light caused by its gravity cannot be detected. 6. The observations only give support to, but cannot confirm, Einstein’s theory. 7. A possible conclusion is that Einstein’s general theory of relativity is wrong or needs to be modified. 8. The true scientific attitude is that a theory would be unacceptable if it fails in certain tests. © Times Publishing (Hong Kong) Limited -3-