English for physicists. Лидия Страутман

Чтение книги онлайн.

      1. There are times when graphing motion may not be the most efficient or effective way of understanding the motion of an object.

      2. To assist in these situations, you can add a set of problem-solving equations to your physics toolbox, known as the kinematic equations.

      3. The scientists must carry out the experiment at once.

      4. The workers had to solve the problem yesterday.

      5. The engineer should be perfectly familiar with the properties of materials.

      VI. Read the text and make up a summary

      Free Fall

      Examination of free-falling bodies dates back to the days of Aristotle. At that time Aristotle believed that more massive objects would fall faster than less massive objects. He believed this in large part due to the fact that when examining a rock and a feather falling from the same height it is clear that the rock hits the ground first. Upon further examination it is clear that Aristotle was incorrect in his hypothesis.

      As proof, take a basketball and a piece of paper. Drop them simultaneously from the same height… do they land at the same time? Probably not. Now take that piece of paper and crumple it up into a tight ball and repeat the experiment. Now what do you see happen? You should see that both the ball and the paper land at the same time. Therefore you can conclude that Aristotle’s predictions did not account for the effect of air resistance. For the purposes of this course, drag forces such as air resistance will be neglected.

      In the 17th century, Galileo Galilee began a re-examination of the motion of falling bodies. Galileo, recognizing that air resistance affects the motion of a falling body, executed his famous thought experiment in which he continuously asked what would happen if the effect of air resistance was removed. Commander David Scott of Apollo 15 performed this experiment while on the moon. He simultaneously dropped a hammer and a feather, and observed that they reached the ground at the same time.

      Since Galileo’s experiments, scientists have come to a better understanding of how the gravitational pull of the Earth accelerates free-falling bodies. Through experimentation it has been determined that the local gravitational field strength (g) on the surface of the Earth is 9.8 N/kg, which further indicates that all objects in free fall (neglecting air resistance) experience an equivalent acceleration of 9.8 m/s² toward the center of the Earth.

      (NOTE: If you move off the surface of the Earth the local gravitational field strength, and therefore the acceleration due to gravity, changes.)

      You can look at free-falling bodies as objects being dropped from some height or thrown vertically upward. In this examination you will analyze the motion of each condition.

      What is gravity? Gravity is the mysterious force that makes everything fall down towards the Earth. But what is it? It turns out that all objects have gravity. It's just that some objects, like the Earth and the Sun, have a lot more gravity than others. How much gravity an object has depends on how big it is. To be specific, how much mass it has. It also depends on how close you are to the object. The closer you are, the stronger the gravity. Why is gravity important? Gravity is very important to our everyday lives. Without Earth's gravity we would fly right off it. We'd all have to be strapped down. If you kicked a ball, it would fly off forever. While it might be fun to try for a few minutes, we certainly couldn't live without gravity.

      Gravity also is important on a larger scale. It is the Sun's gravity that keeps the Earth in orbit around the Sun. Life on Earth needs the Sun's light and warmth to survive. Gravity helps the Earth to stay just the right distance from the Sun, so it's not too hot or too cold. Who discovered gravity? The first person who dropped something heavy on their toe knew something was going on, but gravity was first mathematically described by the scientist Isaac Newton. His theory is called Newton's law of universal gravitation. Later, Albert Einstein would make some improvements on this theory in his theory of relativity. What is weight? Weight is the force of gravity on an object. Our weight on Earth is how much force the Earth's gravity has on us and how hard it is pulling us toward the surface. Do objects fall at the same speed? Yes, this is called the equivalence principle. Objects of different masses will fall to the Earth at the same speed. If you take two balls of different masses to the top of a building and drop them, they will hit the ground at the same time. There is actually a specific acceleration that all objects fall at called a standard gravity, or «g». It equals 9.807 meters per second squared (m/s²).