## Hunus 4 years ago Elastic potential energy

1. Hunus

|dw:1335300188613:dw| When does the spring have negative potential energy?

2. Hunus

Does it?

3. anonymous

Good question! Do you know the expression of this type of potential energy?

4. Hunus

$\frac{kx^{2}}{2}$

5. anonymous

So...

6. Hunus

Well, when you have the equation $F_x=-\frac{\partial U}{\partial x}$ what force does F_x represent?

7. Hunus

Because if F is the force of the spring, then wouldn't it be positive on the third picture? And if it was positive wouldn't dudx have to be negative?

8. anonymous

9. Hunus

I'm reading through a book on classical mechanics and I'm having a difficult time understanding when the force is positive$F_x=−\frac{∂U}{∂x}$

10. mos1635

when F positive dx is negative??

11. anonymous

Draw the function U(x)=½kx² For what values of x is the tangent to the curve positive/negative ? As Fx = -dU/dx, you will know the sign of Fx.

12. anonymous

dU/dx is the derivative of U with respect to x. Its value is the gradient of the tangent to the curve at point (x,U(x)).

13. anonymous

Quote : Well, when you have the equation Fx=−∂U/∂x what force does F_x represent? Answer : it reprensents the force exerted by the spring on the block.

14. Hunus

Thank you

15. anonymous

Welcome :)