High-Q | חטיף 3


חטיף מס '3


Reading Comprehension This part consists of two passages, each followed by several related questions. For each question, choose the most appropriate answer based on the text .

Text I (Questions 13-17)

On the earth, flames are shaped by gravity. The heated gases rise, creating a buoyant airflow that feeds oxygen to the fire. But on the space shuttle in orbit, there is no buoyant flow. Fires must get their oxygen from diffusion - the slow drift of molecules through the air - or from air currents generated by ventilation fans. The NASA Research Center has conducted experiments to determine how flames spread in a microgravity environment such 5 as that encountered in space. These experiments have shed light on the physics of combustion and helped NASA to better identify the risks of fires on spacecraft. Astronauts on the space shuttle studied the burning of plastic and cellulose materials, measuring how quickly the flames spread under different airflow conditions. In a still environment, where the air is not moving, most materials burn more slowly in microgravity 10 than they would on the earth. But with a low-speed airflow - 5 to 20 centimeters per second - certain materials become more flammable instead of less. For example, a sheet of paper under a low-speed airflow in microgravity will burn 20 percent faster than paper under the same airflow conditions on the earth’s surface. This result provided a lesson in spacecraft safety: the first line of defense is to still the air by turning off all ventilation fans. 15 These investigations were only part of a much broader program. Other experiments conducted on the space shuttle focused on gas-jet fires, burning fuel droplets, hydrogen-air mixtures, and the soot produced by fires in microgravity. When completed, the International Space Station will include a fully equipped combustion chamber, offering opportunities for further study. 20


13. The main subject of the passage above is -

(1) studies of combustion in space (2) microgravity (3) the dangers of fires in spacecraft (4) buoyant airflow

14. According to the first paragraph, fire in space differs from fire on earth in that -

(1) it has no defined shape (2) no natural airflow exists to feed oxygen to the fire (3) it can only occur with the aid of ventilation fans (4) it is more difficult to extinguish because it occurs in microgravity

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