# Homework Help: High Speed Camera Problem

1. Nov 16, 2007

### iwalani

[SOLVED] High Speed Camera Problem

Rotational Kinematics

So I'm pretty sure I'm just asking someone to point out the obvious for me, but I've been trying to figure out where to start on this problem for way too long. I don't want the answers, just some guidance on how to start in terms of what is given and what conversions need to be made. Thank you for your help in advance!

Problem:
High speed cameras are used to photograph events which take place too quickly to be seen by the human eye. One such high speed camera is composed of a rotating film drum, a rotating shutter and some miscellaneous optics. The film is fastened along the outside of the drum (along its entire circumference) and the drum is driven by high pressure gas.
You desire to photograph the dynamics of a bumblebee in flight and estimate that its wings flap somewhere about 10,000 times a second. The radius of the film drum is r = 0.501 m and you've loaded it with 35mm film .

Questions:

(a) How many photographs will fit along the circumference of the drum? (Assume 35mm length is one photo)
Hint: One photo fits on every 35mm of film length. The total length of the film is equal to the circumference of the film drum.

(b) You desire to take the photographs at a frequency of 20,000 Hz. What is the angular frequency of the shutter? (Note: shutter has one hole in it!)
Hint: There are 2*p radians per revolution. You desire the solution in units of radians per second.

(c) What must the angular frequency of the film drum be?

(d) When the drum has taken its pictures, the gas is turned off and the drum slows down uniformly, finally setting to a standstill after 2 minutes. What was the angular acceleration of the film drum after turning off the gas?

(e) How many times does the film drum rotate after the gas is turned off and before coming to rest? Give your answers in revolutions.

Last edited: Nov 16, 2007
2. Nov 16, 2007

### catkin

Nobody has rushed in to answer this post probably because:
a) it is formatted too wide to fit on a 1024 screen
b) the lower part of the graphic does not display
c) it is not clear what is the question, what the hints (if any) and what you are musing

Net result: the reader has to do a lot of figuring out and guessing to understand the post before starting on the actual physics pronblem ...

3. Nov 16, 2007

### mgb_phys

A, Actually 35mm film is 35mm wide - frames are 36mm long on a 38mm pitch.

4. Nov 16, 2007

### JonWhitehouse

just some thoughts:
(a) circumference of circle =2*pi*radius in meters. 35mm = 0.035m
(b) frequency of shutter =f= 1/number of times hole passes film = 20,000 hz, not the same as angular freq.
(c) freq. f of drum = 20,000/number of pictures on circumference.
this should give you some raw data to start the next section.

5. Nov 16, 2007

### iwalani

Not sure why the formatting ended up too wide (wasn't on my little computer), but hopefully that is fixed, and hopefully I have edited to successfully counteract the net result stated above. Thanks for the info.

______________________________

Thank you JW for the info, it helps. My issue with physics problems is that I get distracted by all the info in the problem, and it is tough for me to pick out the 'obvious' and pertinent bits, as I am not a natural at physics or math~ (nor am I a photographer for that matter!)

If I have any specific questions I will post again