# How long do a shock last

1. Nov 20, 2012

### MEriksson

Hello, I'm trying to estimate how long the impact of a steel ball hitting a plastic box would be.

The actual case: you drop a steal ball d=0.51mm, m=0.54kg from h=0.74m onto a box made of plastic, the impact speed is 3.86m/s and force 4J. I want to know the deceleration speed during impact or how long the impact lasts so i can use F=ma, to get the other. Anyone have a estimate for these numbers?

The reason: I'm comparing UL standard 60068-2-27 with IEC standard 60950-1 :P

2. Nov 20, 2012

### pukb

impulse = Favg * t = m * v
Favg = avg. impact force
t = time of contact or collision
m = mass of falling ball
v = change in vel of ball after and before collision

it must be assumed if it is an elastic or inelastic collision to find out velocity of ball after collision using conservation of momentum considering losses.

impact force has to be found out somehow. it has to be greater than m* g . it is usually taken to be twice of static force due to mass mg.

from the above data you can find out collision time.

i considered elastic collision.
v = 3.86ms-1 for both before and after collision. hence net change is 7.72ms-1.
Favg = 2*0.54*9.81

gives t = 0.8sec

3. Nov 20, 2012

### Enthalpy

It depends only on the stiffness of both objects if the collision is elastic. The answer is in the low ms range, or lower with your small objects, and the shock often in the 10,000G range. In the plastic range, it depends on plastic behaviour, sure.

Not obvious: the "elastic behaviour" involves generally the propagation of waves in the solids. Seriously difficult to determine with objects that were not designed for their shock behaviour.