CBSE Class 11-science Answered

The answer is based upon the frictional force. Consider the 1/2 kg block kept along with the door, now let us suppose the friction coefficient between the block and the floor is a non zero value, so the door has to overcome the frictional force experienced by the block in order to move it and the spring force easily exceeds that because the maximum value of frictional coefficient could be 1, so maximum the door has to move 1/2 kg block by a force of maximum 1/2*g = 5 N. But when you place the small 20 g wood then the frictional force between the door and the floor comes into picture and in this case, since the mass of the door is much more than 1/2 kg, so the normal force is exceedingly high and hence the frictional force is high that can not be provided by the spring in normal cases. That is why the 20 g wood piece is sufficient to stop the door.

 

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