# Homework Help: Find the ration of the centripetal accelaration

1. Aug 10, 2009

### jandominic145

The large blade of a helicopter is rotating in a horizontal circle. The length of the blade is 6.7m, measured from its tip to the center of the circle. find the ration of the centripetal accelaration at the end of the blade to that which exists at a point located 3.0m from the center of the circle?

i don't know to get 2.2

Last edited: Aug 10, 2009
2. Aug 10, 2009

### Cyosis

Show us what you have attempted so far. Start with the formula for centripetal acceleration.

3. Aug 10, 2009

### jandominic145

centripetal acceleration
Ac = V^2/r

4. Aug 10, 2009

### Cyosis

Yes and you know that the blade describes a circle. So how can you calculate v in terms of the radius and period?

5. Aug 10, 2009

### jandominic145

Ac = 6.7m^2/3m
= 44.89m/3m
= 14.96

the answer is 2.2 not 14.96

6. Aug 10, 2009

### Cyosis

The length of the blade is 6.7m, how can that be equal to its speed? Length and speed are not the same! I will ask again how can you express v in terms of the radius and period?

7. Aug 10, 2009

### jandominic145

i didn't understand it

8. Aug 10, 2009

### Cyosis

What part do you not understand and why are you ignoring my question for the second time? If you are unwilling to cooperate we're not going to get anywhere. I will guide you to the answer, but only if you cooperate.

9. Aug 10, 2009

### jandominic145

60 degrees

10. Aug 10, 2009

### Cyosis

'the circle of degrees is 60' makes no sense to me. It is also irrelevant to the problem. For the third time now I will ask you the following question:

How can you express the speed v, in terms of the radius r, and the time it takes for one revolution T?

11. Aug 10, 2009

### jandominic145

what is T revolution

12. Aug 10, 2009

### Cyosis

As I said, the time it takes for one revolution, the period.

13. Aug 10, 2009

### jandominic145

how?

14. Aug 10, 2009

### Cyosis

Could you be a little bit more specific when it comes to asking questions and giving answers. Single phrase 'sentences' really don't work well to convey a message. I do not see how the sentence you quoted from me can generate the question 'how'? How what?

15. Aug 10, 2009

### jandominic145

the time it takes for one revolution, the period.

16. Aug 10, 2009

### Cyosis

We are having serious communication issues here. Do you know what a revolution is, do you know what a period is? If you do, explain both concepts to me.

17. Aug 10, 2009

### jandominic145

i don't know what is revolution and period

18. Aug 10, 2009

### Cyosis

The blade describes a circle, it rotates. After some time T the blade returns to its original position at that point in time it has done one full rotation called a revolution. The time it takes for the blade to make one circle is called the period. You want to know the speed of the blade at a distance r from the center.

19. Aug 10, 2009

### jandominic145

R is 3 m

20. Aug 10, 2009

### Cyosis

R can be all values in between 6.7 and 0. You have a formula for the centripetal acceleration with two variables r and v. r is known but v is not therefore we want to find v. I have been telling you to do this for the fourth time now.

If you pick a certain point on the propeller blade and let the blade rotate until it comes back to its original position. How much distance has that point traveled? hint: the blade describes a circle

21. Aug 10, 2009

### jandominic145

6.7m

22. Aug 10, 2009

### Cyosis

Great another one phrase answer without any explanation to why you think that is the answer. Are you just guessing or what? The blade is 6.7m LONG and is rotating. I placed a point a distance r from the enter on the blade, never did I say the point was on the tip of the blade.

The blade is rotating it describes a circle do you see this? (answer this question)
Therefore the point on the blade follows the circumference of that circle. How much distance does the point move during one revolution? (answer this question)

23. Aug 10, 2009

### jandominic145

i dunno

24. Aug 10, 2009

### Cyosis

So you don't know a single one of those questions? I must say you don't make me feel like you are actually trying to understand this. So I am going to try one more time.

Do you understand and see how the blade describes the circumference of a circle when it starts rotating?
What is the circumference of that circle?