I'm not sure where to post this... this seemed like the right place but if a mod wishes to relocate this to a more appropriate forum, that's fine by me.

My daughter is doing a science project where she has to design a mars lander that will save an egg from destruction and she has chosen to use peanut butter as her material for shock absorbing.

I wasn't sure this was a great idea so I googled it and from what I was reading you can actually put peanut butter in your shocks to get a more stable ride in your car... at least, that's what's in the list of 1001 uses for peanut butter...

Has anyone ever heard of this and know of the science behind it?

I'm not planning on doing it, it really does NOT seem smart... but I am curious how it's supposed to work.

Thanks!

 

Well, I don't know the science behind it, but is the peanut butter supposed to be solid or more fluid inside the shocks? If it is supposed to be fluid I might give you an answer in two months time when I'll have finished studying for my fluidodynamics course at university

 

I'm not sure, I'm continuing to try to dig up stuff on this through google, but there are so many uses for peanut butter that wading through all these other things to get to what I'm looking for is slow, tedious business.

 

I guess the peanut butter is a material that can compress enough to absorb shock. I wouldn't try somehow putting it in my car's shocks since I'm sure the fluid in them now is far superior for the function across a wider range of temperatures. Probably works well for a science project, on mars with the temperature well below zero at night (and after a very cold space-flight) the peanut butter would probably be about the consistency of a brick. Thinking like this is why I shouldn't have kids, they'd wind up with something that could actually land on Mars. lol

 

My kids would love you. They're both really into physics, I think this is why the daughter decided she could handle this project in two days instead of taking the two months worth of time she had to complete it

 

My guess is that someone says the thickness of peanut butter is similar to the viscosity of some shocks' oil. I also do not think this is correct, more of if you HAD to, peanut butter COULD work in a shock.

Neither peanut butter or oil will compress, not sure about peanut butter, because liquids don't compress. It is probably more of a flow issue rather than compression.

For your daughter it should be how well will the peanut butter displaces to transfer the energy from the egg's travel into the peanut butter's flow. My guess is it depends on the peanut butter.

 

The simplest "make an egg survive a drop" solution I ever saw was to pack the egg in the middle of a can of Crisco. I suppose the peanut butter would work equally well.

The secret is that while the Crisco/Peanut butter won't compress, it *will* move out of the egg's way upon impact. The consistency is just right that it allows the egg to move far enough so that the deceleration isn't very abrupt, but it doesn't let the egg move enough to impact the side of the container.

 

Okay we figured it out how the peanut butter in the egg drop experiment works by applying Newton's Third Law of Motion.

The fluid inside the egg upon impact is pushing down on the shell, but the peanut butter around it pushes back up on the shell with an equal amount of force, thus reducing the amount of force pushed upon the shell by the liquid inside it.

Still haven't figured out what it's supposed to do in shocks... can't find any science to explain that one LOL!

 

I don't know if your daughter is being graded on the explanation as well as the actual apparatus, but it's not really a Third Law issue, it's an impulse (force delivered per unit time) issue. The peanut butter surrounding the egg acts like the "crumple zones" in your car, causing the egg's deceleration to happen over a period of time, rather than instantly. This lowers the g-forces acting on the egg dramatically, and it's these deceleration forces that actually break the egg.

Picture driving in a car that's perfectly rigid and inelastic, and picture yourself strapped securely in the seat. If you hit a wall at 30 MPH, your body is going to decelerate from 30 MPH to zero virtually instantaneously, pretty much guaranteeing you an instant death.

But if your car is designed to crumple on impact, then your slowdown from 30 MPG to zero happens more gradually over the space of a few feet, and you'll likely survive.

 

Great Thread

 

She's in middle school, but she is being graded on explaining the science as part of her poster board. She's taking general science, not a physics class. They haven't covered elastic and inelastic collisions.

Geez... at this point I'm thinking I should make sure I post pics of it for you. My science skills have atrophied since college... I think my physics teacher would be very disappointed in me.

Thanks Scott! *big hugs*

 

Well, since they're not expecting her to get into the nitty-gritty details of what's going on during the impact, all she has to say is that the egg is allowed to slow down gradually as it moves through the peanut butter post-impact, reducing the "shock" on the egg when the container hits the ground. If she understands the "crumple zone" analogy, that would probably help the judges appreciate the real-world application of the theory as well.

I love science fairs. I've been asked to help judge the Monterey County Science Fair on the 28th, and I'm curious to see what the kids will come up with. I just found out today that the categories I'll be judging are "Electronics & Electromagnetics", "Aerodynamics/Hydrodynamics", and "Applied Mechanics & Structures" (so I may even get an egg-drop experiment in that category!)

 

Also, I don't know if your daughter enjoys science or if she's just doing a project because she has to, but if she likes that kind of stuff, here's a quick demonstration that's close to magic for a little kid.

Take a Pyrex container (like a measuring cup, but it's gotta be Pyrex) and put it in a larger container like a mixing bowl. Then start pouring Wesson or Crisco cooking oil into the Pyrex container (the regular old-fashioned vegetable oil, not canola or sunflower oil). Keep pouring until the Pyrex container overflows and the oil starts filling up the larger container.

As the oil level rises in the larger container and starts to cover up the Pyrex container, the Pyrex container will disappear before your eyes! The index of refraction of cooking oil is almost exactly the same as the index of refraction for Pyrex, so once there's oil on the inside and outside of the Pyrex container, you won't be able to see it, since light passes from the oil into the Pyrex and back into the oil without being bent. If there are ink markings on the Pyrex, they'll appear to be "floating" in the oil - you'll see the markings, but you won't see the container itself.

Here's a YouTube video of the concept. The presentation itself is a little weird, but their explanation is sound.

http://www.youtube.com/watch?v=9AoiiYaJgGw

 

Its been a while since my hydraulics/engineering classes, so if I have any bad information here, please feel free to correct me.

-Fluids don't compress, in fluid filled shocks the force of the vertical car movement moves fluid surrounding the cylinder out of the way. When the force is removed, the fluid moves back into the original space.

-The viscosity of the fluid will affect how slow or fast the fluid moves out of the way. IE - if you filled a shock with machine oil you could probably move it through its entire motions with just your hands. Fill it with a more viscous oil or something that is almost approaching semi-solid like grease, and it becomes significantly stiffer as more force is required to push the fluid out of the way. In addition to viscosity the size of the orifice the fluid has to move through during displacement also affects the dampening ability of the fluid. There are some equations that can compute this, but I A) don't remember them and B) don't feel like looking them up right now.

-Depending on temperature, peanut butter is a viscous liquid. In situations where it is warm enough that the fats are in a liquid state, I guess it would work in a shock or hydraulic just like a petroleum based product. Problem is that the viscosity of Peanut butter varies greatly with temperature - at high temperature it is a very runny liquid, and at low temperatures it becomes a solid mass. This would lead to great instability in the function of the system. In addition to this you also have to worry about the acids, salts, and other fun chemicals present in the peanut butter that work good in the digestive process, but not so good in a closed hydraulic system.

Bottom line is that yes, it would logically work for a short period of time assuming you live in an area that has a fairly constant temperature (probably around 70 degrees). Also, you would need prepared for the day when your shock gaskets are broken down by the oxidative chemicals in the peanut butter and every time you hit a bump your car leaves piles of peanut butter on the pavement.

 

Very cool! I just read your post to the kids, they loved hearing about how it could work, but the idea of the car pooing peanut butter made it seem not so appetizing.

 

I would have guessed she was in university given the task, lol

Just to be more precise, liquids don't compress. Gasses are fluids too, and they do compress, and a lot too.

As for the rest of the post, from my (for now) limited knowledge of fluidodynamics I'd say you're spot on

 

So would you have to put a allergy warning on your car if you fill your shocks with peanut butter?

 

ROFLMAO!