Activity/Demo overview:
This physics demonstration involves the buildup of gaseous pressure as liquid nitrogen evaporates inside a metal tube closed by a rubber stopper. The buildup of pressure is eventually enough to overcome the static friction holding the stopper in place, resulting in the launching of the stopper across the room. In lectures, this demonstration is used to explain the concept of recoil based on launch angle, but here it was purely used to discuss the “cannon” aspect of the buildup of pressure resulting in the launching of the stopper.
Main Overview of Instructions:
Before the show
- In rehearsals, find a good angle for the cannon to fire at
- You can alter the launch angle by sliding the rear of the cannon up or down the vertical rod
- You can mark off the angle you wish to use by using some tape to keep the cannon fixed at that angle
- Be mindful that as the angle is increased and the stopper hits higher up on the wall, the risk that it could go into the first two rows increases
- Ensure a small dewar has been filled with liquid nitrogen and place it near the cannon for use in the show
- The dewar does not need to be completely full to do this
- Place the rubber stopper used to plug the cannon near the dewar of liquid nitrogen
- Place the rubber mallet used to hammer the stopper in the cannon near the rubber stopper and dewar
- Ensure that the cannon valve is open
- Open orientation has the handle being parallel to the rest of the piping
During the show/demo
- Tilt cannon back (or pick the front end up off the ground) to pour liquid nitrogen into the barrel without it spilling out
- If spilling the liquid nitrogen on your hands is a concern, you can wear a cryogenic glove, but I do not think this is strictly necessary
- Place the cannon back on the ground
- Place the rubber stopper in the front end of the cannon and hammer it in using the rubber mallet
- Getting the rubber stopper face to be flush with cannon’s tubing is fine
- Close the gas valve
- Closed orientation has the handle being perpendicular to the rest of the piping
- Tilt the cannon’s front end toward the ground and back up fairly quickly
- This will slosh the liquid nitrogen towards the front end of the cannon where the metal will be considerably warmer than the back of the tube where the liquid nitrogen sits
- This will cause the rest of the liquid to evaporate more quickly, speeding up the time it takes to launch the stopper
- This will slosh the liquid nitrogen towards the front end of the cannon where the metal will be considerably warmer than the back of the tube where the liquid nitrogen sits
Main Safety Concerns:
- There is a nonzero chance that the rubber stopper can bounce towards the front two rows of the audience members once it strikes the wall at the far end of the room
- So, it is worth it to mention the audience that the people in these positions should watch their heads
- Sidenote: This demonstration isn’t too noisy, so that shouldn’t be an issue.