Force of Impact

Force = mass x acceleration.

Acceleration = speed / time (in this case)

6mph = 6 * (5280/3600) ft.s^-1 = 8.8 ft.s^-1

Acc = 8.8 * 1000 / 300 ft.s^-2 = 29.33... ft.s^-2

So force = 1800 lbs * 29.33... ft.s^-2 = 52800 lbs.ft.s^-2

(I think! Not used to calculations in imperial measures these days).

How did you calculate your result?

I think that:

"So force = 1800 lbs * 29.33... ft.s^-2 = 52800 lbs.ft.s^-2"

should read:

So force = 1800 lbs * 29.33/32.2... ft.s^-2 = 1640 lbs.ft.s^-2

using mass instead of weight.

No, I think my answer stands. The 1800 pound "weight" is actually an 1800 pound mass (lb_m). It would still be 1800 lb (i.e. 1800 lb_m) in zero g.

I misspoke the values for the answer......we did get the same result as you (52800)

I am not familiar with the units you use so that please specify if the 1800 pounds are mass or weight.

As indicated the force is Mxacc

If 1800 are mass units then F= 1800*V/t where V is the speed in yiur units and t the time of 0.3s.

If 1800 is the weight then the force will be 1800* (V/g*t) where g = 32.185 ft/s².

V= 6mph= 8.8 ft/s thus acc= 29.33 ft/s²

is 1800 mass units then force = 1800*29.33=52799 lbs

if 1800 weight then force = 1800*29.33/32.185=1640 lbs

May be here is the misunderstanding with your boss.

I believe nick name and passingtongreen are right. I worked this in SI units (believe me, I need the practice):

m = 1800 lb = 816.5 kg

υ = 6 mph = 2.71 m/s

t = 0.3 s

F = (816.5 kg)(2.71 m/s)/(0.3 s) = 7377 kg*m/s² = 7377 N

Converting Newtons to lb_f:

7376 N = 1658 lb_f

A bit off due to someone's conversion errors.

What you must calculate is Impulse: Impulse

OP said the mass (or weight) is 1800 lb. If you use that, answer is ~ 52800 lbs.ft.s-² (also called poundal, or was once, you don't hear much about poundal nowadays). 1 lbf = 32.2 poundal, giving 1640 lbf.

If you convert the mass into slug (1 slug = 32.2 lb) answer comes out direct in lbf (= slug.ft.s-²).

Easier in SI units!

Cheers......Codey

thanks for all the answers.....all GA's it definately helped us to verify that our numbers were correct. From the looks of things we started our right, just didnt finish as needed.

thanks for the help

This solution assumes a constant force causing the deceleration. That is rarely the case in the real world unless you are impacting against an energy absorbing device that is designed to stroke at a preset force-distance rate ( like a hydraulic cylinder discharging into a reservoir to lift a dead weight. or a wire pulled through a bending roller). If you are trying to design packaging to absorb impact energy to protect an item from damage you need to look at real world impact conditions and responses. There is a lot of research info on that subject out there in the automotive world and military helicopter crash safety design.