Topic content:
(16 points) A track ABCO is fixed in the vertical plane. The AB section is horizontally placed with a length of 4 m. The BCO section is curved and smooth, and the radius of curvature of the track at point O is 1.5 m; A ring with a mass of 1.0 kg, which can be regarded as a particle, is sleeved on the track, and the dynamic friction coefficient between the ring and section AB of the track is 0.5. Establish a rectangular coordinate system as shown in the figure. Under the action of constant force F in the positive direction of the x-axis, the ring starts to move from a standstill at point A (- 7,2), removes the constant force F when it reaches the origin O, and flies horizontally past point D (6,3). The gravity acceleration g is 10m/s2, excluding air resistance. Ask:
(1) Pressure on the track when the ring reaches the point O;
⑵ The size of constant force F;
⑶ The time when the ring moves in AB segment.
Best answer:
(1) 30N, vertical upward (2) F=10N (3)
Answer analysis:
(1) The ring is thrown horizontally from O to D
(1 point) (1 point)
Reading the drawing, we know that x=6m, y=3m, so v0=m/s (1 point)
When reaching point O: according to the centripetal force formula=(2 points)
Substituting data, FN=30N (1 point)
According to Newton's third law, the pressure on the track is 30N, and the direction is vertical upwards (1 point)
(2) In the process of the ring from A to O, according to the kinetic energy theorem
(3 points)
Substituting the data, F=10N (1 point)
(3) In the process of ring from A to B, according to Newton's second law
(2 points)
According to kinematics formula (2 points)
Substitute data to get time s (1 point)
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.