Subject: swinging

Lately I've been wondering about a very common phenomenon.  When my

little girl swings on her swingset, she uses a pumping motion with her

legs (leaning back and sticking her legs out then leaning forward and 

pulling her legs under her)that makes her swing higher and higher 

without me pushing her.  How is it that she is putting more energy into 

this system so that she swings higher?  How can explain the phenomenon 

in terms of forces?  I have a hypothesis but not the means to test it. 

 Please help

Re: swinging