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NASA has a lot of available teaching material that might work for you. Below is a link you might find useful.
http://search.nasa.gov/search/edFilterSearch.jsp?entqr=0&output=xml_no_dtd&sort=date%3AD%3AL%3Ad1&ud=1&site=ed_collection&ie=UTF-8&client=default_frontend&oe=UTF-8&start=&nasaInclude=Gravity&grade=K-4&filteredSubject=&subsubjectindex=
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