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3. Friction is a force which opposes motion. Much of the energy supplied to a complicated machine with many moving parts is wasted in overcoming this opposition. The energy is converted into useless heat. The amount thus lost can be considerably reduced and the life of the machine prolonged by careful lubrication. This reduction improves the efficiency of the machine. Another way in which friction is reduced is to substitute roller or ball bearings for the earlier type of bearing, in which the surfaces in contact move by sliding. Bearings of these later types are used very extensively in many kinds of modern machinery.It must not be assumed, however, that friction is always a disadvantage. In many instances it is highly important. Belts are often used in conveying power from one machine to another. In the absence of friction, this could not be done. The driving wheel would not run the belt, neither would the belt run the wheel to driver. Without friction, then, belted machinery could not be used. Again in the case of all motor vehicles, friction is essential in the operation of the brake. Were there no friction, there could be no brake. But even this illustration does not give an adequate picture of the importance of friction. The vehicle could not even be started if it were not for the friction exerted between the tires and the road. Nails hold in wood, and screws hold in either wood or metal by reason of friction, without which they would be useless. Walking is difficult enough on an icy pavement, but it would be wholly impossible if there were no friction at all between the shoe and the pavement.(5分)3.1 Friction arises when____.(1分)A. an opposing force of motion exists B. the energy is wastedC. everything is motionlessD. a complicated machine is supplied with energy
It is said that the New York area has more Jews than ______ city outside Israel in the world.(1分)
The sailors on a ship are____; they are paid to do all the work on a ship.
21.Many household accidents could have been prevented, but accidents seem to make news headlines every day. While accidents can happen anytime or anywhere, parents with young children can prevent accidents at home by taking a few steps. First, set your water heater correctly: To avoid burns from hot water, make sure your water heater’s temperature is set below 120° Fahrenheit, or set to “low”. Anything above that temperature can cause a child serious burns within seconds. Second, beware of candles: Statistics show that the small candle flames cause approximately 15,000 home fires a year. If you have young children and pets, stop using candles or be sure to blow them out before leaving a room. Third, recognize danger signals: It is important to pay attention to signs of danger from electrical appliances. If the wiring in your home does not function normally, you need to update it. Potential warning signs that might require an electrician’s inspection include wavering（摇晃的）TV pictures or computer screens, flickering（闪烁的）lights and shocks from appliances. It is impossible to prevent all accidents, so it is necessary to create a fire escape plan. In addition to having a smoke alarm on every floor of your home, draw a simple floor plan of your home. On it, mark two exits from every room, including windows and an outside family assembly point, such as a drive way or parking lot.(10分)21.1 The possible target readers of this passage are ______.(2分)A. grandparentsB. parentsC. grandparents with young childrenD. parents with small kids参考答案：D
23.4 How many subjects has the author mentioned here that he has learnt?(2分)
That they have lied to us all ________ very obvious.(1分)
5. (A)long (B)short (C)wide (D)narrow
Everest is almost ___ the Matterhorn, the highest mountain in .(1分)
23.3 What should we do if a colleague proposes a bad idea?(2分)
3.In this lecture, I can only give you a purely _____ view of how we can live life to the full and make some suggestions about the future.(1分)
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