a child's father pushes the swing to increases the speed of the spring.why?

 

When the force is applied on the swing it will move back and forth through the chain which is attached to a fixed support. But the amplitude of the swing will goes on decreasing after sometime. Because, the friction between air and swing and the chain and its fixed support will slow down the motion of the swing and will eventually come to rest. So, there must be external force acting on the swing so as to keep the swing in motion.