# Trying to compare two different weights during drop test

1. Oct 15, 2014

### Spoolx

Hi guys,
I am working on a project at work where we make part a which weighs 10 pounds and part b which weighs 13 pounds.

Parts a and b both get packaged in a foam container. I have test data on part a that says it will survive drop tests, I was hoping to do analysis to say part b will also survive the drop test without actually having to test part b.
The drop height is 5 feet. The foam has a specification where it takes 90lbf to deflect it .25 inch

My initial calculation was to find the kinetic energy of both, and then divide by the displacement to find the force, however obviously the foam padding adds some sort of damping to the equation. So I really dont know how to solve this problem.
When I do my KE equation I can't figure out how to get rid of the ft from the numerator as I need lbs to calculate deflection of the foam.

Any help is greatly appreciated.

Thanks

2. Oct 15, 2014

### stinsonbr

How much does the foam deflect during the 10lb drop test?

Perhaps you could first assume an amount of foam is required (i.e 1.5 inches needed to stop the object), then use the amount of foam deflection as your deceleration distance, find a, then find the force from your f=ma. Of course this would require a few iterations until the force you find matches up with the force required to displace said amount of foam. And this is also assuming the foam behaves linearly with respect to force and displacement ( 90lbf = 0.25in, 180lbf = 0.5in, 270lbf = 0.75in, ect.), which may not be realistic, but is something you could possibly assume depending on how it acted in the 10lb drop test.

Or maybe this is a gross oversimplification. Let's see if I get torn apart.

3. Oct 15, 2014

### NTW

I'm not sure if my calculations below are right. In any case, a drop of five feet is a substantial drop, and 10 or 13 pounds are substantial masses...

In SI units, 90 lbf are 90*0,453*9,8 = 400 N. Now, the energy associated with a displacement of that force along 0,25 inch = 0,00635 m is 400 * 0,00635 = 2,54 joule.

Now, if you drop a mass of 10 pounds (4,53 kg) from a height of 5 feet (1,52 m) it will collide with the floor with an energy m*g*h = 4,53*9,8*1,52 = 67 joule...

4. Oct 15, 2014

### stinsonbr

One more question: How much foam are you allowed?

5. Oct 15, 2014

### Staff: Mentor

What kind of parts are they? Is their construction (shape, size, material, etc.) identical except for the weight?