如图所示，质量为10kg的物体，在水平地面上向左运动，物体与水平地面间的动摩擦因数为0.2，与此同时，物体受到一个水平向右的拉力F=20N的作用，则物体的加速度为（g取10m/s2）（ ）
A．0
B．2m/s2，水平向右
C．4m/s2，水平向右
D．2m/s2，水平向左
在水平地面上向左运动，竖直方向受重力、支持力，水平方向受水平向右的拉力、水平向右的摩擦力．
水平向右的拉力F=20N，摩擦力f=μN=20N，
所以合力大小为F合=（20+20）N=40N，方向水平向右，
根据牛顿第二定律得：a=
=4m/s2，方向水平向右，F合 m
故选C．
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| Accidents can happen indoors. Many young children get 1 when playing in their homes. 2 do these accidents happen? A housewife is a 3 person. She has to do most of the work in the home. Sometimes she is 4 . She puts a pot of hot water on a table. A little child can reach it. When she goes out of the room, the child reaches up and 5 the pot. The hot water spills over him and hurts him. Then the child must 6 to hospital at once. Many other accidents happen. Kerosene heaters (加热器) are sometimes 7 by children playing in the home. The kerosene spills out of the heater and burns. Soon the whole room is burning. All things that can cut are dangerous. Sharp knives and scissors should 8 out of reach of children. Small children should not be allowed to 9 them. Electricity is always dangerous. Electricity can be used for heating and cooking, but it can also kill us! You should never touch any electric heaters or wire if it 10 . Medicine makes us 11 or kill us! Sometimes grown-up people are 12 . They leave medicine in places where children can reach it. They find it and eat it. Then they get very ill. Perhaps they die. The skins of bananas, oranges and other 13 are very slippery (滑的). You 14 throw them on the ground. If you do, someone may step on them and fall down. They may break a leg or an arm. To stop accidents, please be 15 in everything you do. | ||||
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