Let’s look at the second calculation question
16. (9 points) A certain grain depot uses a motor with rated voltage U = 380V and internal resistance R = 0.25Ω to transport grain. As shown in the figure, the cables of the
counterweight and the motor connected to the trolley are parallel to the slope. The trolley filled with grain moves up the slope at a constant speed at a speed of v=2m/s. At this time, the current I=40A. After the motor is turned off, the speed of the car is exactly zero when it goes up the slope L to reach the grain unloading point. After unloading the grain, give the car a downward initial speed, and the car will go down the slope at a constant speed. It is known that the mass of the trolley m1 = 100kg, the mass of the grain on the trolley m2 = 1200kg, and the mass of the counterweight m0 = 40kg. Taking the acceleration of gravity g = 10m/s², the frictional resistance encountered when the trolley moves is proportional to the total gravity of the trolley and the grain on the trolley. The proportional coefficient is k, and the counterweight never touches the ground, excluding the mechanical friction loss of the motor itself and the quality of the cable. Find: the proportional coefficient k value of
(1); the upstream distance L value of
(2).
analysis: Faced with this kind of sports-related problems, everyone must focus on three directions in daily training. One is the combination of kinematics and with dynamics. In other words, the combination of Newton's laws of motion and with kinematics formulas. . The second is to look at the problem from an energy perspective. The third is to look at the problem from the perspective of momentum. Taking this question as an example, starting from the first angle, because it is moving upward at a constant speed, the power is equal to the resistance. In other words, the sum of the pulling force generated by the counterweight and the pulling force generated by the motor should be exactly equal to the resistance and the division of gravity in the direction of the incline. force. Speaking of this, do you have a sense of deja vu? Is this the simplest model of a block on an inclined plane that we usually learn? From the second perspective, the energy output by the motor is equal to its internal energy and its output energy. Is this the simplest model? If you start from the third angle, the ontology is not suitable. Under normal circumstances, the third angle will appear time, this physical quantity or a collision-type collision model.
For the second question, it is also the most basic question. To find the displacement, we already know the initial velocity. We only need to find the acceleration. Then the displacement can obviously be calculated quickly. Of course, we can also start from the perspective of energy. Calculation, but the simplest one should be from the kinematic point of view, which is also the most basic model. So when we look at this question, it is a combination of the three most basic models. The more models are combined, the more difficult and complex it feels, and the greater the psychological pressure. Therefore, everyone should also pay attention to this aspect in daily training. First, they should pay attention to being proficient in each model. Second, they should quickly identify the models in the questions. Third, they should improve their ability to withstand pressure.
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