加速度的瞬时变化问题（Instantaneous change in acceleration）.doc

加速度的瞬时变化问题（Instantaneous change in acceleration） Instantaneous change in acceleration Liyang nandu senior high school physics preparation group Cases. (Shanghai 2001) as shown in figure 4 (a), a quality of m in length of l1 and l2 two thin line respectively, one end of the l1 suspended from the ceiling, and vertical direction Angle is theta, l2 level straight, object in a state of balance. Now the l2 line cut, cut the instantaneous acceleration of the object. (1) the following is a solution to the problem: Solution: set l1 line pull to T1, l2 wire tension is T2, gravity is mg, and objects balance under three forces: T1cos theta = mg, T1sin theta = T2, T2 = mgtan theta The moment of the cut, T2 suddenly disappears, and the object is getting acceleration in the opposite direction of T2, because theta is theta = ma, so acceleration a = gtan theta, which is going in the opposite direction of T2. Do you think the result is correct? Evaluate and justify the solution. (2) if the as shown in figure 4 (a) of the fine line l1 to the same length, regardless of the quality of light spring, as shown in figure 4 (b), other things being equal, solving steps and (1) are exactly the same, namely a = gtan theta, what do you think of the result right? Please explain why. Error analysis: the subject test is used in analysis of Newtons law and instantaneous acceleration, instantaneous force students to elastic rope and rigid line the nature of the two kinds of ideal model to make the right judgments, unable to the change of the elongation of cord force does not need to change the length of the rope, thus its elasticity can change in an instant, the change of elastic rope must can only be achieved by changing the length of the rope, thus its elasticity can change in an instant. There is an error in the examinee is usually did not notice the difference between the two kinds of model, two situations will be confused. Level of thinking inspiration: fine lines cut instantly pull mutation is