How to measure force of impact inside container? I am in 7th grade and for my science fair project, I need a way to measure the force on a dropped object when it hits the ground.  What I am trying to determine is which packing materials provide the best protection for an object in a collision. So I am planning on dropping containers filled with different packing materials surrounding some sort of force-measuring device in the middle.  But I don't know how to either obtain or construct the force-measuring device.
Because I am measuring the effectiveness of the packing material, I need to measure the force inside.  One method I thought about was having a metal ball sitting on top of clay. After hitting the ground, the ball will dig into the clay.  I can measure how deep the impact is and assume that the deeper the hole, the greater the force.  But I am not sure if this will work.
Does anyone have any suggestions on how to measure force (using either my idea or something else entirely)?
 A: That sounds like an excellent idea.
You could also test the idea of how deep it goes into the clay by dropping a ball from different heights and see if twice the height = twice the depth into the clay.
Shipping stores sell shock indicators which are little plastic tubes with paint in them that will change color at a certain shock level - but your plan to make the shock sensor yourself would be a better way of showing a physical principle at work. 
Good luck.
A: Perhaps have a pencil attached to the end of a spring inside the container, and a piece of paper the pencil can draw on. Then you only need to check the maximal extension (end of the plotted line) and use Hooke's law ($F=-kx$) with the spring constant (which you can measure by test weights)
A: This sounds like a great project and I think you are on the right track. 
There are hundreds of engineers working on these problems all over the world to estimate what forces crash test dummies experience in car crashes. A standard way to measure forces is using a spring, the more the spring extends or is compressed the higher the force. In your experiment this force will only act for a very short time, probably less than a second, so measuring the length of the spring in that time is hard to do. 
There are at least two possible ways out, either attach a pen to the spring as Guy Ziv pointed out or use a 'spring' that once it is compressed does not go back to its original form. Clay might be a bit stiff, so I would experiment with different materials and balls. If the dent in clay is too small something like Jell-O might work better (the professional term is ballistic gelatin but that uses the same ingredients).
To support your argument an egg might also be useful but not for actual force measurements.
