1. I don't really get the definition of "Potential energy", on my book, it says The energy that an object has owing to its position or condition is called potential energy. Why does it relate to position? (I know the distance involved in formula) Can you give me a definition that is easy to understand. In my opinion, i think it means an object has stored an energy and has the potential to move but it hasn't. Btw, how can i know if an object has potential energy or not. 2. In Elastic potential energy,Hooke's law: F=kx. k is known as the spring constant. I don't get what spring constant means? The resistance of the spring? My teacher said an ideal spring doesn't have mass. WHY? I think every object has a mass? WHy it is called 'ideal' spring? 3. I represent vector in bold. Fel=-kl Why here there is a negative sign? IN the study of mechanics, the sign of a vector is usually determined by the direction? Up and right is positive, down and left is negative? Thanks a lot for answering.