# Spring stiffness k, in drop test.

#### Michael777

##### New member
Hello simulators,

I am trying to simulate a drop test from 30 ft. for a flashlight which weighs 1.085 lbs., body ABS (Zytel ST- 801).

I am using some assumptions for the model :

a/no initial velocity

b/single degree-of-freedom

c/no energy dissipation, all kinetic energy is absorbed at impact by the unit.

d/the object remains attached to the floor by a spring after initial impact

d/ the object is a single mass, vibrating at its natural frequency.

I am using the eqn. : x(t)= a sin(ωn*t)

Where ωn=sqrt(k/m), where m is the mass and k is the spring stiffness.

What would be a good starting value for k?

TIA,

Michael

#### Luis Aguirre

##### New member
Hello Michael,

First of all impact is a nonlinear analysis due to the contact and disipation of energy in the model and bivration modes exited. However if you want to do a first order approximation here is what you can do. Find the location in your 3D cad model where you assuming is going to hit the ground. Create a contact region of impact and apply a load normal to the surface. Hold the model in a location away from the region of the impact. Run a static analysis and find out how much did the model deform in the direction of the load. After that use load you applied in the impact region and divided by the total displacement obtained from the static analysis run. Again this assumes that this spring is linear but in reality this is not the case. I hope this helps

Luis