随手关门会减速

In normal times, you maybe have such an experience, when sb pushes and closes the open door which has a lot of clothes on its back, he/she may find out that at the end the door begins to get slow, why?

 

for Conservation of Momentum

 

at first, the door and the clothes both has the same speed, in the course, the clothes undertakes the horizontal directional gravity, its momentum has increased, so the door's momentum has then decreased, so does its speed.