# How to solve this pendulum problem?

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1. Jun 8, 2017

### Helly123

1. The problem statement, all variables and given/known data

For number 3,4,5

https://s4.postimg.org/qbp3xzq65/IMG_1092.png

https://s22.postimg.org/u220j60sx/IMG_1093.png

2. Relevant equations
Centripetal force = m v^2/R
T = centripetal force + w
Sigma F = m.a

3. The attempt at a solution
Number 3. Find the v at D point using mechanic energy eternity
While kinetic energy at A point is zero, and the potential energy is max (height max)
So, 1/2mvD^2 + mg.hD = 1/2mvA^2 + mg.hA
i dont know the height at D point....

After i find the v at D
I can find T
Which T = centripetal Force + weight
T = mvD^2/R + m.g

Number 4 almost the same
The string doesnt bend (what exactly it means...?)
So, centripetal force > weight??
m.vD^2/R > m.g ...(1)
But why the centripetal at point D has opposite direction than weight force? Is it cuz the string stuck at the nail at C point??

Find the v at D using mechanical energy eternity then add v to ...(1)

Number 5 i still dont know...

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• ###### IMG_1093_1.png
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Last edited: Jun 8, 2017
2. Jun 8, 2017

### BvU

Hello Helly,

Unfortunately your pictures don't show up in your post. I get:

Can you try again ?

3. Jun 8, 2017

### Helly123

ok.. thanks for report. I've changed the pic.. :)

4. Jun 8, 2017

### BvU

with $v_A = 0$.
make a sketch of the situation and you know it. Note that CB = CD.

5. Jun 8, 2017

### haruspex

You know the length of the string, and you know how far C is below A. How far above C is D?

6. Jun 8, 2017

### Helly123

is the height of D start from C or B ........???

CD = a-b
BC = a-b
if D start from C then height of D = a-b
if D start from B then height of D = 2(a-b)

7. Jun 8, 2017

### BvU

Check your energy conservation equation: If A is at height zero, you must take height of D wrt the same referernce.

8. Jun 8, 2017

### Helly123

if A = height zero.. then kinetic energy is max....
then B point the kinetic energy is zero... how can it be...

9. Jun 8, 2017

### BvU

It was you who chose height A = 0 ! Therefore at point B the height is -a and the energy conservation still reads $${1\over 2} mv_B^2 + m g h_B = {1\over 2}mv_A^2 + mg\,h_A \quad {\rm or} \\ {1\over 2} mv_B^2 - mga = 0$$

10. Jun 8, 2017

### Helly123

While kinetic energy at A point is zero, and the potential energy is max (height max)
hmm I see..
then it said the string doesn't bend, how that means Sir??

11. Jun 8, 2017

### BvU

As long as you have tension in a string, it does not bend but remains taut.
First work out number 3, once you have that, then number 4 is easy

By the way, what did you get for number 2 ?

Do you realize image 1093 is extremely fuzzy ?

12. Jun 8, 2017

### Helly123

number 2? I get sqrt 2ga. do you even know the questions Sir.. but I'm still trying for number 3...

13. Jun 8, 2017

### BvU

Very unsharp, in image 1093 under (2) It says: Find the magnitude of the tension in the string just before the small ball reaches B.
$\sqrt{2ga}\$ is not among the choices. But also, 2mg ...
Is the wrong answer (as I already suspected ) .

You need to get this right: the same issue also needed in (3).

14. Jun 8, 2017

### Helly123

lol.. sorry Sir. I meant 3mg..

15. Jun 8, 2017

### BvU

Agree with 3mg.

16. Jun 8, 2017

### Helly123

oh.... it's my mistake... lol i'll try again number 3.. how do you even know the question for number 2, from the pic there's no number 2

17. Jun 8, 2017

### BvU

18. Jun 8, 2017

### Helly123

19. Jun 8, 2017

### BvU

I know that.

20. Jun 8, 2017

### Helly123

I don't even understand... How to solve number 3....

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