Magdeburg hemispheres - safety precautions

[DDesulgon]

I would like to show students the Magdeburg hemispheres and ask for two volunteers to try to separate the hemispheres. In doing so, however, I am concerned that if the students actually succeed, they may fly off to the back and possibly injure themselves.
How would you avoid this - for example, is it an effective way to have the students simply brace themselves against a table with one arm? Or would that do little or make the danger only worse?
Thanks in advance!

 

[berkeman]

How big are your hemispheres? What force have you measured that is required to separate them? You might be able to have the students hold onto something stationary with one hand, and use the other hand to pull on the sphere handle...

Demo with makeshift "hemispheres":

https://makezine.com/projects/the-magdeburg-hemispheres/

 

[DDesulgon]

They are quite small, with a diameter of about 10 cm. I have not measured the force; two strong men can separate the hemispheres with some effort.

Do you think it makes sense for both of them to hold onto a table with one hand? Or is the recoil then so strong that this rather leads to an injury of this arm?

 

[berkeman]

Do you have any tables or other things that do not move? Like lab benches used for experiments? Or do you only have desks and tables that can move?

 

[DDesulgon]

Yeah, I have this kind of immovable experiment benches.

 

[berkeman]

Another option would be to have the two students lock hand-to-wrist with their free hands, to help them push back while trying to break the vacuum, and to help them retain their balance if it does break apart.

 

[DDesulgon]

To be honest, I'm rather skeptical about that. What do you think about the benches?
Or a completely different idea: Students sit on the floor and brace their feet against each other while pulling on the hemispheres. Or can the head hit the floor then?
Maybe I'm worrying too much, but I'd rather play it safe....

 

[berkeman]

How old are your students?

 

[berkeman]

BTW, you could ask for 4 volunteer students, and have the extra 2 students behind the pullers as "safety" helpers...

 

[DDesulgon]

They are roughly 16. I have thought about the "safety" students, too - I don't know what's best, really ;) What's your favourite?

 

[berkeman]

Do you have access to a room with mats? Like what the wrestling team or martial arts team uses for practice? If there are mats underfoot and you have the 2 "safety" students, it seems like it should be pretty safe.

 

[DDesulgon]

No, unfortunately nothing like that.

 

[berkeman]

Alternately, maybe sitting across from each other at a picnic bench, with the 2 safety students behind the pullers...

 

[DDesulgon]

That seems a bit cumbersome for me as well. But I'm amazed by the amount of your ideas ;)

 

[berkeman]

Thanks. Safety is important to me; in addition to working full-time as an EE, I also work part-time as a medic. So preventing injuries is a big part of what I do. (Less paperwork for me if I can prevent an injury before it happens...)

 

[DDesulgon]

That's interesting! It seems that I have found the perfect person for my question :)
In summary, which variation would you favor for classroom implementation?

 

[berkeman]

If you can do the seated picnic table version with 2 "helpers/safety people", that seems to have the most advantages. If not, then 4 people standing at the corner of one of your fixed lab tables, with the 2 "pullers" using their free hand for extra pushing force on the table (and providing a bit of a brace if the spheres separate), and the 2 "helpers" behind with a light hand on the puller's shoulders in case they fly backward.

 

[berkeman]

Let us know how it goes!

 

[kuruman]

Attach one end of the spheres to a fixed and sturdy vertical support at one end. Make an equal arm balance by attaching the midpoint of a two-by-four at the other end of the spheres. Have two students of (more or less) equal mass grab onto each end and lift their feet off the ground. The force pulling on the spheres will be the sum of their weights plus that of the two-by-four which is most likely greater than what they can exert horizontally using static friction. If the spheres separate, the students can safely extend their legs and touch the ground. Repeat using a pair of heavier students.

 

[gmax137]

I was going to say, come-a-long and a load cell or spring scale. But I like the idea of the kids putting their own muscles into it. I like @kuruman 's idea.

 

[berkeman]

I was going to say, come-a-long and a load cell or spring scale.

Agreed. I would have measured the required force early on in the planning. And probably would have done a number of trials in case there was a lot of variation (in the quality of the seal).

 

[jrmichler]

Attach one sphere to a bar or 2X4. Student pushes against bar with feet while pulling on the other sphere with hands. When the spheres pop apart, action and reaction leave the student in place. No flying students, so no need for safety people.

It's important to adjust lengths so that student pulls with arms straight and legs slightly bent. If legs are straight and arms bent, then student could punch themself in the face. If legs are fully bent (squat position), then student gets poor leverage.

I like the idea of measuring force, but spring scales can be damaged if the load is suddenly released.

 

[malawi_glenn]

BTW, you could ask for 4 volunteer students, and have the extra 2 students behind the pullers as "safety" helpers...

I do this, use saftey students

 

[Filip Larsen]

How about a small internal rope between the two half so when they separate they only move very short distance?

 

[FinBurger]

Or better, a short chain or rope connecting to the two pulling eyes, that is long enough so it is hanging slack. Then everybody can see that the chain is not doing anything when the hemispheres are together. And if they do separate, the slack chain will only allow a short movement.