NASA Science Space Place. (May 4, 2017). What is gravity?. Retrieved Aug 10, 2019, From https://spaceplace.nasa.gov/what-is-gravity/en/
Greg Kestin. (Aug 5, 2014). What Is Gravity Made Of? A "cosmic fingerprint" reveals the universe's beginning and gravity's microscopic secrets. Retrieved Aug 10, 2019, From https://www.pbs.org/wgbh/nova/video/what-is-gravity-made-of/
The Institute for the Promotion of Teaching Science and Technology (IPST), Ministry of Education, a non-profit organization under the Thai government, developed SciMath as a website that provides educational resources in Science, Mathematics and Technology. IPST invites visitors to use its online resources for personal, educational and other non-commercial purpose. If there are any problems, please contact us immediately.