The light spring is upright and on the horizontal table. The object a with a mass of 2kg is placed on the top of the light spring in a static state. Now the object B with a mass of 3kg is placed on the table What is the elastic force of a to B at the moment when it is placed on a?

Analysis: first of all, when B is put up, the spring force will not change, which is 20n. Analysis of a and B shows that they are subjected to 50N downward gravity and 20n upward elasticity, so the overall acceleration is 6m / S ^ 2. Therefore, the downward acceleration of B is 6m / S ^ 2. Then analysis of B shows that they are subjected to 30n gravity and f upward. Finally, the acceleration is 6m / S ^ 2. The calculated elastic force is 12n and upward

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