Gifted Issues Discussion homepage Forums General Discussion Where to look for an answer

Took the kids to the circus last night. DS6 was fascinated by the fact that a 550lb lion could sit on a 50lb table. He said the table should come crashing down. I tried very briefly to explain weight displacement etc.

Per usual my answer wasn't good enough. Where would I look, what subject would I find a detailed explanation under?

Hi, Shari,

Not sure if the answer is in here, but this book has tons of similar questions. I think I first read about it here on this forum: http://www.flyingcircusofphysics.com

Not sure how you explained it. How detailed do you think you need? Do you know any engineers/physics types that could draw him a force diagram? Did you try explaining that it depended on the strength of the table materials (at some point the table wouldn't hold the lion? An example at home might be a cheap overloaded bookshelf with the shelves bowing in the middle (we had lots of those when I was growing up).

Science4kids has a Level 1 Physics book that might have some information on forces/strength. I couldn't tell from the sample.

The way this works is that, assuming the lion is in the middle of the table, his weight will be evenly distributed to all four legs of the table. (137.5 lbs per leg). According to Newton's 3rd law, (every action force has an equal but opposite reaction force), as long as the table legs and sturdy enough to push back up on to the lion with 137.5 lbs of force, he will be safe. However, if the legs get wobbly, or a chunk gets taken out or they somehow get damaged the amount with which they can "push" back up on the lion, to keep him on the table will decrease, and at that point the table will crash down with him on it. Now, if the lion is at one end of the table, he is presummably large enough to fill most of the end of the table, so his weight would basically be distributed over 2 legs (although there would be a bit on the other legs too), so in theory, each leg of the table must be able to hold up 275 lbs, or there is a chance of it crashing down on the lion.

(I teach physics, and this is a classic problem that students have problems with all the time.) If you need any more details, pm me and I'll draw a diagram and write an explanation as a word document and send it to you.

Hope this helps - this is my kind of question.

I thought maybe he needed the strength of materials answer as to why the table top doesn't buckle. It seemed the OP had tried the distributed weight explanation. That's why I wondered what details were throwing him with the original explanation and wondered if he was wondering why it didn't buckle. This is more complicated than a simple force problem right? That's why I mentioned buckling bookshelves. it's been a long time since I've done any equations though.

There are two mechanical properties called stress and strain. There are two types of stresses - static and dynamic.

The short answer is that the stresses ( dynamic and static ) of the load of the lion did not produce enough strain in the table to break the table.

There is an idea called strength-weight ratio or specific strength.

http://en.wikipedia.org/wiki/Specific_strength

Here is a pretty good article on stress and strain.

http://en.wikipedia.org/wiki/Stress_(physics)

And then good old statics:

http://en.wikipedia.org/wiki/Statics

Thanks everybody!! I think with all of your help, I've appeased the beast for now. Physics type questions seem to be coming up a lot lately. Not my strong suit to say the least!

I'm considering doing some sort of Physics with him over the summer. I'm looking at bitesize physics and Physic I through noeoscience. Any feedback?