Peak force of falling object

  • #1
I'm running an experiment where I drop a mass (99.7kg) a distance of 1.8m. The load is suspended from a very rigid structure through a steel cable.

I currently have a load cell in series with the cable and mass. I acquire data at 1000Hz and my load cell tells me I have a peak force of about 6000lb.

I'd like a way of verifying the peak load I'm reading through my load cell.

This is what I have so far:
Potential energy = mgh = 1760 Joules
To calculate load, I thought I would need to calculate the distance the steel cable stretched after the load dropped. This is where I have difficulty. I found a utility that tells me the stretch of my cable for a given weight. It tells me I will get a stretch of 0.0006989049 meters on my 9ft cable.

Using Work = Force x Distance,
Force = 1760 / 0.000698 = 2518940N = 566280 lb, which is obviously wrong.

Now I'm thinking there is a couple of problems with my calculations:
1- The stretch distance is not accurate because it does not take into account that the weight was dropped from a height and has momentum
2- The weight bounces back up after initial impact and keeps bouncing back up and down several times before the weight comes to rest.

Any way to do this calculation with the information I have?
 

Answers and Replies

  • #2
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I found a utility that tells me the stretch of my cable for a given weight. It tells me I will get a stretch of 0.0006989049 meters on my 9ft cable.
That is the stretch in equilibrium, with ~1000 N force, not with the ~30,000 N you have. If you increase the distance by a factor 30 then the force becomes ~100,000 N, that is at least in the right order of magnitude.

The force won't be uniform across the stopping distance and your stopping process is of the order of the 1 ms sampling frequency, so I wouldn't expect a good match anyway.

The bouncing shouldn't matter if you just consider the process until the weight is at its lowest point.
 

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