Simple way to measure fps using a camera and cardboard. your video camera takes a picture several times per second, if you know how fast id dose this then you can calculate speed my camera takes a picture 30 times per second (30 frames per second) if your camera is slower it will be less accurate and might not see projectile.
I shot my 180 fps airsoft gun and my camera got only 1frame of it. My 270 fps gun the camera did not see.
this is not to be judged in the launch it competition, its for the many people that could use this to improve their instructables
video on step 5
using some gestimation (fired sling shot and half way pulled back) i estimated the speed of my sling shot at 72 miles per hour (at full speed the camera didnt see any thing so i just multiplied by 2 i know its not accurate but it dosent have to be.)
the pics below:
it spent 4 frames on the lined part of board. after crunching the numbers the 14 gram projectile averaged 7.5fps (this is one of 5 tests) it was fired from my 7 gram torsion spring catapult
Step 1: Gather Materials
you will need:
a camera (at least 20 frames per second)
flat cardboard ( around 2 feet long 1 foot tall)
ruler (as long as cardboard is tall)
tape or thick marker (tape: blue scotch painters tape 1inch think)
calculator (recommended not needed)
Step 2: Board Set Up
using a pencil and ruler mark 1 inch along the board
do the same for opposite side.
then draw a strait line across.
if using tape, tape right between the lines.
if using marker color between lines
if your board has little bit left over just put "spots" so you know its not a full inch
Step 3: Set Up.
I used a table vice to help hold card board in place (I didnt use the vice part of it through)
You could tape the board to a wall
I had to put a bucket of putt behind the board so it wouldnt tip over (I used putty because it was right sitting on the bench behind vice)
Keep in mind the angle of trajectory, catapults tend to fire at a high angle, sling shots k'nex guns airsoft guns Ect. Go relatively strait (duh)
Step 4: Set Up Camera and Projectile
you will need a tri-pod to get a good video
i used a heavy weight (2x as much as the catapult its self) this way it would go slower and would be easier to document for instructable.
Step 5: Fire/lanch/shoot/setoff/throw
Zoom in so you only see the board.
Get ready load/cock your weapon
Start filming, quickly fire your weapon into a safe backstop stop the camera
REPEAT 4+ times to rule out anomalies
Step 6: Computation
After you have taken the videos record the data
Go frame by frame in video,
1Count the number of frames the projectile flew over the board not the entire time its visible only when its on the marked part of board # will be referred to as A
2Record the distance the projectile flew over the board it should go strait across but the first test mine didnt # will be referred to as B
3And last, record how many inches it traveled in the fist frame # will be referred to as C
Now average every thing up so youre just working with the 1 set of what you did above. So you have one A, one B and, one C
The speed the camera records will be referred to D
The simple way I did this was CXD/12=Feet per second
EXSAMPLE 3X30=90 90/12=7.5=feetpersecond
So know you know approximately how fast it was traveling
Want to convert FPS to MPH?
just multiply the fps by 0.6818 and your done!
want to convert to something else
check this out