# Creating an impact

1. Mar 29, 2007

### mrb112103

Hello,

I am probably going to suffer some embarassment, once I see the answer to my question. Anwyay, I need to calculate the distance needed to create a 5ft-lbs. impact with a 1.18lb. sphere being dropped vertically onto a surface.

How would I go about calucating the vertical distance needed?

This is for an impact test from UL, where the mass and impact force are defined, but the distance is not.

2. Mar 29, 2007

### denverdoc

(well I abhor those english units, but I reckon the principles are the same). what i like even less are words like "impact." since its in ft-lb, I assume they are looking for energy.

there are a few ways to go about this, but usually the most direct is to equate potntial and kinetic energies

wt*h=1/2Mv^2=impact energy

3. Mar 30, 2007

### mrb112103

Still unclear...

So if the Mass of the sphere (M) = 1.18 lbs = .535kg & the impact force = 5 ft. lbs. = 6.8 Joules, then is the velocity = to acceleration by gravity ~ 9.8m/s^2?

So it's .5(.535kg * (9.8 m/s^2)) = 6.8 Joules ??

4. Mar 30, 2007

### Dick

If your you want your final kinetic energy to be E, then use E=mgh. Or like denverdoc said, E=F*d (force times distance). Gravitational force is 1.18 lbs, you want 5 ft-lbs of impact energy, so d=E/F=(5/1.18) ft.