Now for something a little bit different. I don’t know if you ever did this experiment in school, but it was a classic for young science students, back in my day. (Kids, please don’t do this without a responsible adult).
1. Take a finished syrup tin (good excuse for lots of pancake eating), wash the tin out, then place a small amount of water in the tin and put the lid on firmly.
2. Place the tin on a stove / gas burner. Check that there’s nothing to be broken when the lid comes off. Light the stove, stand back & watch.
3. As the contents of the tin heat up, the water will boil & become steam. These actions will steadily raise the pressure within the tin. At some point the internal pressure will overcome the lid’s resistance and the lid will fly off the tin at quite a rate – it’s been given some Kinetic Energy.
So I initially thought it would be fun to recreate this childhood experiment & perhaps film it at a moderately high framerate. But then curiosity got to me and I wondered if we could estimate the speed at which the tin lid moves; perhaps we could even estimate it’s energy level.
Below is the edited & narrated video produced from the Nikon B700 @ 120fps and the GoPro @ 240fps:
So now we need to collect together our measurements & do some calculations:
Distance (d) from top of syrup tin to top of camera frame = 17cm=0.17m (measured from centre point of lid)
Frame Rate of Camera = 240 fps
Number of frames for tin to leave frame view = 2 frames (As carefully viewed frame by frame; we were lucky with the timing of the lid pop).
Time taken (t) = 1 / (frame rate / no. of frames) = 1/120s = 0.0083s
So using the simple formula for speed: s=d/t
Call it 20 metres per second. In imperial units, that’s about 45 mile per hour.
Kinetic Energy=0.5mv2 (expressed in kg & m/s)
Mass of syrup tin lid = 8g
So 0.5*0.008*202=1.6 Joules of Energy
That’s in the same order of energy as a farm electric fence pulse, so don’t get hit by that flying lid, (our fence line runs a minimum of 3 Joules).
For comparison a high velocity bullet for a small calibre rifle (.22) might have a mass of 2g but a muzzle velocity of 500m/s. That extra velocity packs a big punch because we’re talking v2, so:
0.5*0.002*5002=250 Joules of Energy
So in summary, we did manage to estimate the lid velocity at ~ 45mph and also its energy level of ~ 1.6 Joules. Enough to leave a decent splatter on the studio ceiling!