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It's been a while since physics 100, but I'll take a stab at this.
Your units are screwed up. You can't use 500 pounds (US) and 9.80m/s^2 (metric) for gravity. Well, you can I guess but it doesn't make sense.
You need to first calculate the speed at impact.
Velocity = the square root of 2*9.8m/s^2 * height (I had to look that one up)
So, velocity = 4.84 m/s
Momentum = m* v = 1292 kg m/s
Calculating the impact force is more difficult since you need to know what happens when the weight stops. F=ma is true, but the "a" isn't acceleration due to gravity, it's deceleration at impact. In other words, you have to know how long (in time) the impact lasts.
If we assume (and this is one hell of an assumption) the impact is one millisecond, which might be true if nothing is deformed in the impact.
Then the force of impact is 1292280 Newtons, which is 290,000 pounds of force.
The rub here is that it's highly dependent on the time of impact. Double our time to 2 miliseconds, and you cut the force in half.
To be honest, I really didn't think it would be this complicated when I started. Hope I didn't screw it up.
Interesingly enough though, this is why recoil pads on guns and shock absorbers work - the force is directly related to the amount of time over which that force is applied. If you can double the time, you cut the force in half.
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