题目

People have known for a long time that plants can hear, see, smell and communicate with each other. Now, they have been recorded making sounds when stressed.In a study, Itzhak Khait and his team found that tomato and tobacco plants can make ultrasonic( 超声的 ) noises. The plant “cry out” due to lack of water, or when they are cut. The sound is just too high for human to hear.Microphones were placed 10 centimeters away from the plants and picked up sounds in the range of 20 to 100,000 hertz( 赫兹). Human hearing usually ranges from 20 to 20,000 hertz. On average, “thirsty” tomato 15．如图所示，一个重为5N的大砝码，用细线悬挂在O点，现用一水平力F（可改变大小和方向）拉砖码，使悬线偏离竖直方向37°时处于静止状态．（1）若所用拉力F水平，求拉力F的大小；（2）若要所用拉力F的最小，求拉力F的大小和方向；（3）若所用拉力F大小为4N，求拉力F的方向．