Question

In lecture 18 of physics I 8.01 (walter lewin), at minute 31, he mentioned that UB (potential energy) of a spring is equal to - 1/2 KX^2
Why the negative sign??

Answer 1

I dont want to look at the video as it will use up my downloads , I have watched them before.

If you compress a spring then you store potential energy in it ( the (1/2)kx^2 ) , he was most likely stretching a spring, thats the only way you could get possibly get a negative sign.

Answer 2

I disagree.

Even if you stretch the spring (what he was in fact doing in the example, btw), the potential energie has to be positive. \[U_e(x) = {1 \over 2}kx^2 \]As you can see in the definition, it is positive for negative x-values, too - cause of the square...

Lewin also does say it correctly @30:43 (as well as several other times) and when plugging it in @31:55 he writes down a negative sign. I think it's just a slip-up and you don't need to worry about it.

Answer 3

That's what i also thought...Thanks a lot:)