Re: Why is Jello a good shock absorber?

"I am doing a science fair experiment to determine if an egg dropped in 
jello, foam, towels or feathers will break.  The egg dropped in the jello 
from 3 feet and 6 feet didn't break and I am challenged with finding out 
why jello absorbed the shock and didn't break the egg". 



Hi Lily,
Thanks for the very interesting question!

There are two general things that the “shock absorber” (what ever it is) 
must do to save the egg:
1.	It must slow the egg from its top speed (which is just before 
impact) to zero- very smoothly and evenly.
2.	It must change its shape to conform to (match) the contacting 
surface of the egg.

To understand each of these things, let’s look first at about the worst 
shock absorber- a concrete floor:  

When an egg hits the concrete, there is zero time for the speed to change 
from maximum to a stop.  This produces enormous forces and “stress” on 
the egg shell.

To make matters worse, (as per #2), the concrete will not conform at all 
to the shape of the egg.  This means that the egg is stopped by a tiny 
spot on its surface that first touches the concrete.  The ‘rest’ of the 
egg is still moving downward causing the shell to crush Humpty Dumpty 
style.  (This is not much different from hitting an egg on the edge of a 
bowl to open it.)

Now let’s look at one of your other shock absorber options- the 
feathers.  Feathers would do an excellent job of saving the egg- probably 
the BEST of all the materials IF the feather bed was deep enough.  If you 
used only about 2 inches of feathers, they would do a great job of 
slowing the egg, and conforming to the shape.  But although the egg would 
slow, when it compressed to the bottom of the stack it would have to 
instantly stop, and the large forces would still crack the shell.  If the 
stack of feathers was 4 feet tall, you can see that the egg would be 
slowly and safely stopped.

Ok, now after looking at what happens at the two extremes- concrete and 
feathers, let’s look at the Jello:  The egg will survive the Jello 
landing because the Jello should meet both the #1 and #2 requirements.  
At first contact, the Jello must “get out of the way”, or “displace” to 
make room for the incoming egg.  This displacement of the Jello absorbs 
the energy of the falling egg and evenly slows it to a stop.  Also, as 
the egg sinks in, the Jello will conform to the shape of the egg and give 
it support (unlike the concrete).

The main reason that the Jello behaves as described is because of 
inertia.  Inertia is the resistance of an object to move (or to stop 
moving).  Inertia is related to the weight of an object.  A piece of 
Jello has weight (or more accurately ‘mass’) about equal to water.  When 
we talk about the mass of a material per volume, we have defined the 
term “density”.  (Bear with me will all of these terms and definitions).

The density of our shock absorber materials is an important factor that 
will determine how fast the egg will stop.  The low density of the 
feathers does very little to stop the egg.  The feathers act more like 
springs that compress to stop it.  BUT the Jello has a much higher 
density, and therefore higher inertia to resist and slow the falling egg.

Here is another idea that might help you to understand the concepts of 
density and inertia and how they work to stop the egg:  Jumping into 
water.

If you step off the deck of a pool and jump in you feel very little 
resistance from the water.  But if you jump off of a high diving board 
you will feel a strong impact on your head or feet or worst- your belly 
when you hit the water.  The difference in each case is the speed that 
your body hits the water.  The faster the impact speed, the more the 
inertia of the water resists moving away to let your body enter.  An 
extreme case is someone jumping off of a high bridge into water.  People 
can be killed if the bridge is high enough because the high impact speed 
makes the surface of the water act more like concrete than water!  

This explains some of the science of how the jello shock absorber will 
work.  

Answering this question got me wondering- will an egg ever break if it is 
dropped into a pool of water from a high enough height?  I think that it 
would break, but I am not sure.  I might just do a little experimenting 
myself!

I hope that this answers your question.

Best Regards,

Jay Shapiro