# Angular Speed Question,

1. Apr 17, 2006

### mcolem

I have this question I am stuck on and I don't understand and need help:

In a neutron velocity selector, two thin discs of a neutron absorbing material are rigidly mounted on a shaft 1 meter apart and rotated with constant angular speed of 1000 revolutions per minut. There is a slot in each disc and the angular seperation between slots is 3 seconds of arc. What would be the velocity of the neutrons which pass through this system?

2. Apr 17, 2006

### Staff: Mentor

3 seconds of arc is what fraction of a revolution? (1 revolution = 360 degrees.)

3. Apr 17, 2006

### mcolem

I know that 1000 rev/min is about 105 radians/second. The frequency is 16.7 Hz and the time would be 0.0599 seconds. But, I don't know how to find the angle.

4. Apr 17, 2006

### Staff: Mentor

Start by answering my question. Look it up if you have to.

5. Apr 17, 2006

### mcolem

It would be 50 revolutions every 3 seconds.

6. Apr 17, 2006

### Staff: Mentor

"seconds of arc" is a measure of angle, not time:

1 revolution = 360 degrees
1 degree = 60 minutes (of angle)
1 minute = 60 seconds (of angle)

7. Apr 17, 2006

### mcolem

So, if one revolution is 360 degrees and 1 degree is 60 minutes, then 3 seconds would be 3 seconds/3600 seconds = 8.333e-4 degrees?

8. Apr 17, 2006

### mcolem

I dont quite know what formula to use. Is it V= omega x radius or arc length= velocity x time? Do I add the velocity of both discs or is it just on velocity?

9. Apr 18, 2006

### Abhishek.S

By the time the neutron travels from one disc to the other, the disc should have rotated by 3 seconds.(Here 1 second = 1/3600 degrees.)
The disc makes 1000 revs. per min. which implies it rotates at the rate of 6000 deg. per sec. i.e. 6000*3600 seconds(angle) per sec.
Therefore it takes 1/(7200000) secs. to rotate 3 by seconds.In this time it travels 1m.Therefore its velocity is 7200000 m/s.