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# Physics/force

Question
QUESTION: My son wants to do a project in which he wants to measure the amount of force needed to break different bones? How do we demonstrate that project in science lab?

ANSWER: Human bones or animal bones?  You can do that in the lab or not in the lab pretty easily.  Force doesn't break bones, though, shear does.  Torque is more important than force.  The concepts may be a little advanced.  Are you thinking of experiment or theory...or both?  Give me something more to go on and I can help you do something appropriate.  Do you want to break some beef bones with a weight and a support apparatus, for example, and approximate all bones as the same thing?

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QUESTION: I won't be able to find human bones. My son who is in 6th grade is interested in finding about the force needed to break bones and he has to relate that project to an  invention in the last 100 years. I was thinking of using beef and chicken bones and dropping weights from some suppoert apparatus. He can use the acceleration of gravitational force and weight as mass, then calculate force. He needs to demonstrate it in the science lab.
As for invention he is thinking of relating broken bones to X ray development.
Any thoughts???

ANSWER: I wasn't suggesting you find human bones.  I was suggesting you break chicken bones, like you were thinking.  You can't drop weights to make them snap, you have no way of measuring how much shock they exert.  It should be the slow addition of weights to pull on the bone.  Beak a small bone, and then you can scale it up by its cross-sectional area.  Human bones are about an inch or two across.  If you measure the diameter of the bone you break and multiply the weight it takes to break that by (d1/d2)^2 where d1 is the diameter of human bone and d2 is the diameter of bone that you break.  The clamps might be a little involved to rig up, but shouldn't be impossible.  Wear eye protection.  It will be much easier if you don't try to pull them apart, but you try to shear them (Far less weight).  As for x-ray development, I'm not sure what you mean.  He can't work with x-rays, he's a minor.

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QUESTION: Thanks for your help and guidance but I am a little confused now. So you are suggessting breaking chicken bones by putting weights on it and measuring the diameter of that bone. Then multiplying the diameter with weight to get the shear force. How would I translate that info to human bones? Also would he need some kind of spring scale to measure the weight or suspend the weight? how will this formula d1/d2^2 help?
As for Xrays he will not be using any xrays. He has to show how his experiment is linked to some recent development in the last 100 years. I was thinking that xrays can be connected to this project by showing different kinds of fractures and may be some ideas on protective equipment to protect sports related injuries.

Thanks again. Appreciate your help.

You have to multiply the force you use by the ratio of the diameters squared, because the force should be proportional to the cross sectional area.  You can just use weights, if you know them.  But to get a force f2 (what it would take to break a human leg) you have to multiply the force f1 (what it takes to break the chicken bone) by that ratio.  i.e. f2=f1(d1^2/d2^2)

Physics

Volunteer

#### Dr. Stephen O. Nelson

##### Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

##### Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Education/Credentials
Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.