Spring fitted doors close by themselves when released. You want to keep the door open for a long time, say for an hour. If you put a half kg stone in front of the open door, it does not help. The stone slides with the door and the door gets closed. However, if you sandwich a 20 g piece of wood in the small gap between the door and the floor, the door stays open. Explain why a much lighter piece of wood is able to keep the door open while the heavy stone fails.

Question

Philoid · Accepted Answer

Both the cases are illustrated above. In first case a stone is kept in front of door to keep it open and in second case the wood piece is kept between the air gap of door and floor.

In second case, the piece of wood develops greater friction as compared to first case. It is because force of friction depends upon the normal force developed by the object but not on the weight of the object. The normal force provided by a stone is equal to its weight itself i.e. half kg. but in second case, the piece of wood is under the door and hence the normal force developed in this case is much higher as compared to first one. Hence, the small piece of wood could stop the door from closing.