This should cause us to attach great importance and attention. Therefore, we must in the usual teaching and college entrance examination review process, the image type (linear, sine, cosine, hyperbolic, parabolic), the physical meaning of the image (such as displacement image, speed Images, wave images, vibration images, etc., image widening, deformation, interchange, application of images, etc. must increase the intensity of training, so that students can deepen understanding and comprehensive mastery, and smoothly solve such problems for students. The problem lays a solid foundation. At the same time, using images to solve problems is intuitive, vivid and vivid. Sometimes it is simpler and clearer than solving problems with other methods. It has its irreplaceable advantages. Therefore, it is of great significance to cultivate students' ability to use images to solve physical problems.
The above four scenarios are different. There are equilibrium states such as 12 and non-equilibrium states such as 3, but no matter which scenario, the spring can be used as the research object. Obviously there is FF"=ma. It can be seen that for the light spring, since m=0, there is F=F" regardless of whether a is equal to zero. The spring force in the four figures is F, and the same spring, the elongation Must be equal, the answer is D.
From the above analysis, the quality of the light spring can be considered to be zero, so that the elastic force at both ends is always equal regardless of the state of the spring. Example 2 As shown, a spring scale is placed on a smooth horizontal surface, the mass of the outer casing is m, and the mass of the spring and the hook are not counted. Applying horizontal forces F1 and F2 to generate acceleration a in the direction of F1, the reading of the spring balance is A.F1B.F2C.
12 (F1 + F2) D.ma analysis From the previous analysis, the tension at both ends of the spring is F2, and then the third law of Newton knows that the force of the spring pull shell is equal to F2. For the outer casing from Newton's second law, F1-F2=ma, F1>F2. However, the reading of the spring balance is related to the elongation, which is equal to the elastic force of one end. So the answer is B. It can be seen that, under the premise of neglecting the quality of the spring, regardless of the state of the spring balance, the reading is equal to the magnitude of the pulling force of the scale hook.
When the two ends of the spring are connected to the object or one end is fixed to the other end of the object, the spring length cannot be abrupt due to the inertia of the object, so the spring force of the spring cannot be changed. As shown in the figure, the horizontal rope and the light spring jointly fix a heavy ball. . When the spring is at an angle to the vertical direction at rest, the horizontal rope is now cut, and the acceleration of the ball is sought at the moment of the break.
When the horizontal rope is not cut, the heavy ball is subjected to the action of gravity mg, the spring pulling force F1 and the horizontal pulling force F2, and is in an equilibrium state, at which time the two component forces G1 and G2 of gravity are balanced with F1 and F2, respectively. As shown in (a). At the moment of cutting, since the position of the ball is not changed, the length of the spring is not changed, that is, the deformation of the spring is not changed, and F1 is unchanged. Gravity and elastic force F1 are unchanged, that is, the two forces of gravity are not changed in direction, and G1 is still balanced with F1, as shown in (b). G2 is the external force of the heavy ball. According to Newton's second law, the direction of acceleration should be in the same direction as G2, that is, horizontal to the left, and the size a=G2m=mgtanm=gtan.
In the above question, if the spring is replaced by a string, the situation is different. As shown, the force of the ball before the horizontal rope is not cut is the same as (a). At the moment of cutting the horizontal rope, since the elastic force of the light rope is generated by a small deformation, it can be abruptly changed. When the horizontal rope is cut, the ball is centered on the hanging point and is placed along the arc, that is, the state of the ball changes. The ball has an acceleration along the tangential direction. At this time, the effect of gravity mg is to produce acceleration along the tangential direction, and the second is to balance with the tensile force F"1. The force analysis is as shown. The small ball acceleration a=G2m=mgsinm=gsin. The direction is perpendicular to the rope. Arc tangential direction.
When the end of the light spring is disconnected from the object, the sudden change of the elastic force is zero. As shown in the figure, a weight m is suspended under the spring, and then fixed on the ceiling with a string. After the whole device is balanced and stationary, the string is blown with flame. . At the moment the rope is broken, the acceleration of m is a (ignoring the spring mass and air resistance). Then: Aa
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