# Homework Help: Can you help me in this homework?

1. Sep 19, 2007

### coconut88

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

Aboy whirls aball on a string in ahorizontal circle of radius 0.8m. How many revolutions per minute must the ball make if the magnitude of its centriptal acceleration is to be the same as the free-fall acceleration due to gravity g?

2. Relevant equations

3. The attempt at a solution

2. Sep 19, 2007

### learningphysics

3. Sep 19, 2007

### PFStudent

Hey,

Consider what acceleration is being referred to here, centripetal or linear?

Also, then consider that acceleration as it relates to the number of revolutions per minute.

Thanks,

-PFStudent

4. Sep 19, 2007

### Kushal

the best way to solve these kinds of problems is to find the appropriate equations and determine which of the quantity has to be common in the equations(usually two equations).

then equate these equations to find the unknown, which should be in one of the relevant equations.

5. Sep 19, 2007

### coconut88

a=v2/r this is the first equation that I used but I don't kow what is the secend will be ?
what does the revoluition mean ? what does the equation we need to use ?
pleace help me I me international student fot that I have a lot of diffeculties

6. Sep 19, 2007

### learningphysics

Yes, you can find v using a = v^2/r

number of revolutions means number of times it goes around in a circle...

let n = total number of revolutions. let d = distance. let t = time.

So

$$d = n*2{\pi}r$$

$$\frac{d}{t}= \frac{n*2{\pi}r}{t}$$

v = d/t

number of revolutions per second = n/t

Using these you should be able to get number of revolutions per second. What do you get?

7. Sep 20, 2007

### coconut88

I used this v^2/r and the answer is 2.8 m/s after that I used 2rr and the answar is 5.0/1 rev that means 1 rev/5.0
aftet that I can't do any thing because I don't have the d !!
??

8. Sep 20, 2007

### learningphysics

you don't need d. let X = number of revolutions/second

you know that v = X*(distance per revolution) = X*5.0

so v = X*5.0

solve for X.

9. Sep 20, 2007

### coconut88

The x will be .56

10. Sep 20, 2007

### learningphysics

looks right to me. 0.56 revolutions/second

11. Sep 20, 2007