A _________ can be used as a door stopper.

Hint: In order to understand this question we need to understand Newton’s First Law which states that any particle which is in state of motion or rest wants to be in the same state unless acted upon by unbalanced force. So in order to stop something we can apply external force although applying external force forces the body to change its state but the body shows resistance to it known as inertia.

Complete step by step answer:
So according to Newton’s First Law, to stop the motion of a door which is rotating around its own axis we need to apply external force. These are some common examples of external force that we can use:
(i) Use our hand to apply force against the door in order to stop it. But this method had a disadvantage as we cannot stand there all day against the door because in doing so our energy would eventually lose and we would no longer have the ability to stop it.

(ii) We can use a rubber stopper mechanically with a door which when lowered down it touches ground after some time motion ceases and door stops. This happens because of friction, when rubber which is fitted with a door touches the ground and drags for some length then frictional force acts on it in the opposite direction bounding the rubber to stop which in turns binds the door to stop. This method is more convenient than using a hand because rubber deformation is temporary and it can withstand its shape for a longer duration of time.

Hence, a rubber stopper can be used as a door stopper.

Note: It should be remembered that the floor under rubber must be rough and high coefficient of kinetic friction but if it is not so then we have to use such a method which does not have any contact with ground. In that case we can use a rope tied across the handle of the door and the handle on the wall around which the door is moving because in that case motion of the door cannot occur beyond the highest tension of the rope and we can stop the door for a long time.