## Hunus 3 years ago Elastic potential energy

1. Hunus

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

2. Hunus

Does it?

3. Vincent-Lyon.Fr

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

4. Hunus

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

5. Vincent-Lyon.Fr

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. Vincent-Lyon.Fr

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. Vincent-Lyon.Fr

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. Vincent-Lyon.Fr

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. Vincent-Lyon.Fr

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. Vincent-Lyon.Fr

Welcome :)