Topic content:
(1) Briefly describe the movement law of the elevator within 0~25s.
(2) Find the maximum pressure on the elevator floor.
(3) Draw the relationship between the pulling force F and time t in the coordinate system when the elevator is rising.
Best answer:
(1) 0~5s, uniformly accelerated rise;
5. ~ 15s, rising at a constant speed; 15~25s, uniform deceleration rise (2) 510N
(3)
Answer analysis:
(1) 0~5s, uniformly accelerated rise;
5. ~ 15s, rising at a constant speed; 15~25s, slow down and rise evenly.
(2) 0~5s, the pressure in the rising stage of uniform acceleration is the maximum, and the acceleration in this process is calculated as
①
For goods, according to Newton's second law, ②
Solution ③
According to Newton's third law, the maximum pressure on the elevator floor is 510N.
④
(3) As shown in the figure
Scoring criteria:
(1) 3 points, (2) 7 points, including 2 points for the ① ② ③ formula, 1 point for the ④ formula, and (3) 3 points, 13 points in total
Test core:
force analysis:
1. Methods of force analysis of objects ① Methods: overall method, isolation method.
② Choice: When the physical problem involved is the interaction between the whole and the outside world, the application of the overall analysis method does not need to consider the role of internal forces; When the physical problem involved is the interaction between objects, the isolation analysis method should be applied, and then the internal force of interaction in the original whole will become the external force of each independent object.
2. The sequence of force analysis: first gravity, then contact force, and finally other external forces.
3. Problems that should be paid attention to in force analysis ① When analyzing the force on the object, only the force exerted by the surrounding objects on the research object is analyzed;
② During force analysis, do not apply too much force or omit force. Pay attention to determining the strength object and force bearing object of each force. In the synthesis and decomposition of force, do not regard the actual non-existent resultant force or component force as the force applied to the object;
③ If the direction of a force is difficult to determine, it can be analyzed by hypothesis method;
④ The force condition of the object will change with the change of the motion state, and it can be determined by calculation according to the learned knowledge when necessary;
⑤ The external effect of force analysis should be considered as a whole, and the interaction should be isolated.