# Q: How to measure the force of impact of 100,000 lbs/sq inch or more?

## Main Question or Discussion Point

I have a question for you:

My father designs and builds post pounders (machines similar to pile drivers use to "pound" wooden fence posts into the ground). He has had great success building new and innovative pounders and would like to know just how hard they are capable of hitting (very handy when it comes to R&D and further improving his design).

Lets assume that we were to manufacture an artificial "post" that was, for all intents and purposes, immovable (say heavy walled steel, driven into bedrock and cemented), to act as our control (right?).

Question: How would you go about accurately measuring the force of impact?

We think we know an approximate figure, +/- 110,000 lbs/sq inch - but this conclusion was likely drawn up on the shop floor with chalk. The only method I have conceived was to "pound" a known quantity of specific metal/alloy (lets say, a 10x10x10 cm lead block) against our "control" post and calculate the impact based on deformation. Then again, much of my knowledge of physics comes from the Big Bang Theory and Wikipedia - and hence don't know if one can deduce the force this way.

We have tried calculating the force mathematically but in my mind there are just way too many variables within his design, furthermore we lack the equipment to accurately measures these variables (like rate of acceleration or mass for example).

Any ideas? Thank you so much. Cheers everyone.

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Simon Bridge
Homework Helper
The term "impact force" has no meaning in physics.
Impacts take a finite amount of time during which some energy and momentum is exchanged.
The force typically varies during this time - so there is no one "impact force".

In a collision, a force-time graph looks like in inverted parabola.

A totally immobile and rigid ideal post would have an impact duration of zero to produce a finite momentum change - which would give an infinite force. So that's probably not a useful model.

Usually impacts are often investigated using accelerometers.
You can characterize how hard the impact was using the specific impulse - which is the area under the first bounce of the f-t graph.

You could also put a standard post-head (you don't need the whole post) on top of a heavy spring, strike the post and see how far the spring compresses - maybe against a ratchet. You can easily find out how the spring behaves under known blows and so create a scale to compare the new systems with.

meBigGuy
Gold Member
Is 100,000 lbs/sq-in really the right way to think about it? For example if you pounded on a 1 ft diameter post, the thing you are trying to measure wouldn't be any different than pounding a 6 in diameter post. I think what you really want to know is the momentum transferred to the post. The change in momentum is force X time.

The spring idea above is good. Actually just measuring the acceleration of a large weight on a spring would give you repeatable measurements. Rig up a geared dc motor or linear actuator as a generator that will spin as the weight descends. Measuring the acceleration of 500 lbs of iron might work.

Another way might be a lever based system where you measure the acceleration of a heavy weight at the end of a long lever. Using a 10:1 ratio lever is like the strongman hammers at the fair. The elastistity of the lever becomes an issue, but may be consistant from run to run until fatigue sets in.

Or, if you can set it up sideways you can measure the acceleration of a large weight. You could also measure the acceleration of a large weight downward if it was rigged to release when struck.

There are pizeo systems that can measure huge impacts. http://www.pcb.com/TestMeasurement/Force/impact#.Ul-P6j8gmt0 can handle up to 50,000 lbs - maybe they know a novel way to apply it in your application.