remembering how a string works: when you pull on it$,$ it stretches. Newton's $3$rd law says that the spring pulls back on your too when you pull on it$.$ if you pull the end of the string in the $+$x direction with a force F in the $+$x direction, the spring stretches by an amount x$.$ this, then, gives us the equation. where k is the force constant of the spring. what are the physical dimensions $($units$)$ of the force constant, k$?$ what does newton's $3$rd law say about the force that the spring must exert on your hand? $($in one dimension you can show direction with just a $"+"$ or $"-"$