## Jasminebunnies 2 years ago Conservation of Mechanical Energy: A pendulum bob is released from some initial height such that the speed of the bob at the bottom of the swing is 1.9m/s. What is the initial height of the bob?

1. imron07

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2. Jasminebunnies

Why would K(A) be 0?

3. imron07

Yes, it 'released' without initial speed right?

4. Jasminebunnies

isnt the initial speed 1.9 m/s?

5. imron07

No, that's the speed at B.

6. Jasminebunnies

oh ok so you got the zero from A?

7. imron07

Yup :)

8. Jasminebunnies

but the book says tha answer is 0.18m but how would you get that answer?

9. Jasminebunnies

*the

10. imron07

I got the same answer. I wrote the formula at the picture:$h=\frac{v^2}{2g}$

11. Decart

when it comes to a pendulum does it not depend on the lenght of the string?

12. Decart

U=mgl

13. Jasminebunnies

ok, so you would use that formula, h=v^2/2g to get 0.18m?

14. imron07

@Jasminebunnies : Yes

15. Jasminebunnies

ohhh ok and the zero is put there because there is no mass?

16. imron07

Omg! (^ ^!) Which zero?

17. Jasminebunnies

lol! the zero in your original equation that you said came from A, in front of mgh

18. imron07

That's because v=0 at highest point.

19. Jasminebunnies

ohhh ok, is that because it has not been released at that point?

20. Jasminebunnies

@imron07

21. imron07

Because by the time the pendulum is released, the velocity is zero. It start from rest.

22. Jasminebunnies

yes, ok! thank you! I understand now :)

23. imron07

ok ;)