แหล่งอ้างอิง

https://www.slideshare.net/   PhirompornNorachan1/ss-65435668

http://www.krukird.com/L04_2_63.pdf

https://www.scimath.org/lesson-physics/item/7226- equilibrium

https://web.facebook.com/photo/?fbid=1285752014796253&set=pcb.1285756478129140