Physics query

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floydbloke

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#157146 21-Nov-2014 08:22

After 20 years or so of being frustrated needing a crowbar to dig holes in the clay soil here in Whitby, yesterday I discovered the existence of these tools. I haven’t used it in anger yet but after a first short trial run on a tree root it appears to be pretty effective.

I’m curious now as to the difference in impact/force/pressure when it hits the ground in comparison to a standard wrecking bar.

For comparison’s sake let’s say

each tool weights 10 kg,
the internal weight of the ground breaker in the picture is 2kg,
the width of the blade of the ground breaker is 100mm, width of the blade of the standard wrecking bar is 50mm,
the thickness of the blade is 5mm in each tool
each tool is swung with the same speed or dropped from the same height and hits the ground at 90 degrees.

Anyone here with sufficient physics knowhow and a bit of spare time and energy to work this out?

= > ÷

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#1180545 21-Nov-2014 08:45

More to do with inertia and how the energy is released rather than the amount of energy released I suspect.
When the weight hits the stop, then all that energy is released almost instantly with a small high velocity crack rather than a lower velocity thud, the former more efficient at cutting into the ground at the tip, that latter likely to have energy dissipated away from the tip.
I've been digging holes in clay with a heavy bar - and still have the blisters. A cheap SDS drill with a wide chisel bit (about $120 from Bunnings) was useful - but hard to use when holes needed to be ~1m deep.
If I saw that tool for sale - I would have given it a go.

Amosnz

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#1180567 21-Nov-2014 09:10

Cool kiwi invention.

Last night on Grand Designs I saw a large mallet being used that employed the same principle of a movable weight inside the tool. It meant you didn't get any bounce-back when you hit something and thus was safer and easier on the body.

Speedtest

wasabi2k

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#1180607 21-Nov-2014 10:15

Yeah - the amount of energy involved is the same - you lift and swing it. No magic will change that amount of energy input.

However the release of the energy appears to be affected - all that energy released in less time will mean more force.

same principle as car crumple zones, just in reverse.

SaltyNZ

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#1180637 21-Nov-2014 11:04

It's a hammer. You don't hammer in a nail by placing the hammer on the head of the nail and then pushing. You hammer in a nail by swinging the hammer and hitting the nail. Your arm gets to add kinetic energy to the accelerating hammer for the entire duration of the swing (amplified by the fact that the hammer is at the end of a longish handle, and therefore has a higher linear velocity (kinetic energy proportional to velocity squared) than your hand. All that energy is released into the nail in a split second. Impulse rules apply; your hand exerts relatively little force to increase energy during a long swing. The hammer exerts an extremely high force for a very short duration as it rapidly decelerates over a distance of the few mm it pushes the nail.

iPad Pro 11" + iPhone XS + 2degrees 4tw!

These comments are my own and do not represent the opinions of 2degrees.

floydbloke

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#1180642 21-Nov-2014 11:11

Thanks all, the replies all make sense.

So to rephrase my original query, is there a way to tangibly quantify how much more efficient using the ground breaker is vs a normal crowbar?

= > ÷

kiwiyan

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#1180667 21-Nov-2014 11:44

I guess the picture you posted is of Terrax. There is a local groundbreaking ingenuity called the The Slugger a multi tool very handy. This works on the same sliding hammer technique.

http://www.stuff.co.nz/business/unlimited/10292053/Groundbreaking-tool-finds-Pacific-markets

http://www.slugger.co.nz/

ckc

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#1180679 21-Nov-2014 12:00

So it's like a slide hammer for your garden?

gzt

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#1180716 21-Nov-2014 13:01

floydbloke: Anyone here with sufficient physics knowhow and a bit of spare time and energy to work this out?

Not me. At the analogy level this is like hitting an 8kg bar/bollard with a 2kg hammer at a good clip vs whacking a 10kg bar into concrete as fast/hard as you can by hand. But I have a clue, not an explanation. The slide will achieve greater acceleration and therefore generate more force than a standalone bar.

Bung

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#1180719 21-Nov-2014 13:06

kiwiyan: I guess the picture you posted is of Terrax. There is a local groundbreaking ingenuity called the The Slugger a multi tool very handy. This works on the same sliding hammer technique.

http://www.stuff.co.nz/business/unlimited/10292053/Groundbreaking-tool-finds-Pacific-markets

http://www.slugger.co.nz/

The Slugger is hardly an "invention", it is a variation on a theme. There have been slide hammer tools around for years eg www.slidehammers.com . There are quite a few firewood splitters using slide hammers.The Slugger does seem to have a better way of changing just the tip.

I'd have to try the Terrax before buying. Normally you wouldn't be holding the bar tightly at impact so whether the internal shock absorber is better than the external slide would be interesting.

floydbloke

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#1180724 21-Nov-2014 13:17

kiwiyan: I guess the picture you posted is of Terrax. ...

Yeah that's right, althought I've bought the Cyclone brand.

This is a NZ invention also. Or at least it is according to the sticker on mine, and my colleague at work who got me onto this who spoke to the designers at the Mystery Creek A&P show a few years back.

= > ÷

jaymz

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#1183285 26-Nov-2014 13:42

floydbloke:

Yeah that's right, althought I've bought the Cyclone brand.

This is a NZ invention also. Or at least it is according to the sticker on mine, and my colleague at work who got me onto this who spoke to the designers at the Mystery Creek A&P show a few years back.

Curious, where did you get yours from? I am looking to buy something similar.

floydbloke

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#1183372 26-Nov-2014 15:12

jaymz:
floydbloke:

Yeah that's right, althought I've bought the Cyclone brand.

This is a NZ invention also. Or at least it is according to the sticker on mine, and my colleague at work who got me onto this who spoke to the designers at the Mystery Creek A&P show a few years back.

Curious, where did you get yours from? I am looking to buy something similar.

Bought it at Carters in Porirua.

Update on use, I have now used it to dig out a couple of old plants/shrubs and am suitably impressed with the ability to hack trhough roots and get into the hard clay soil. Difficult to quantify, but I feel that I have been able to achieve in 10 minutes which would have taken up to an hour with a spade/saw/axe.

= > ÷

Geektastic

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#1183437 26-Nov-2014 16:26

Amosnz: Cool kiwi invention.

Last night on Grand Designs I saw a large mallet being used that employed the same principle of a movable weight inside the tool. It meant you didn't get any bounce-back when you hit something and thus was safer and easier on the body.

Snap On have made dead stop mallets for years. They use a material that has a soft face but the head is filled with lead shot or similar.

You whack something and the hammer just 'stops' - very strange at first, but effective.

Geektastic

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#1183439 26-Nov-2014 16:30

floydbloke:
jaymz:
floydbloke:

Yeah that's right, althought I've bought the Cyclone brand.

This is a NZ invention also. Or at least it is according to the sticker on mine, and my colleague at work who got me onto this who spoke to the designers at the Mystery Creek A&P show a few years back.

Curious, where did you get yours from? I am looking to buy something similar.

Bought it at Carters in Porirua.

Update on use, I have now used it to dig out a couple of old plants/shrubs and am suitably impressed with the ability to hack trhough roots and get into the hard clay soil. Difficult to quantify, but I feel that I have been able to achieve in 10 minutes which would have taken up to an hour with a spade/saw/axe.

Did you try a tracked excavator?

My late father once wanted to plant about 18 rare rhododendrons and, knowing that I have the knack for driving big machines, asked me to pop home one weekend and dig the holes for him with a 360 he hired..!!

I also once visited a friend who lived on the outer edge of London and mowed his lawn with a hay mower...! I was working on a dairy farm not far away and it seemed a good idea. I cut the whole lawn in one pass (it wasn't a huge lawn) but his wife was not too pleased with the tractor tyre marks across the floral border....

Sidestep

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#1183444 26-Nov-2014 16:34

Geektastic:
Amosnz: Cool kiwi invention.

Last night on Grand Designs I saw a large mallet being used that employed the same principle of a movable weight inside the tool. It meant you didn't get any bounce-back when you hit something and thus was safer and easier on the body.

Snap On have made dead stop mallets for years. They use a material that has a soft face but the head is filled with lead shot or similar.

You whack something and the hammer just 'stops' - very strange at first, but effective.

"Dead Blow Hammer" is a slightly different deal.

Transfers the inertia of the strike but not all through one point. Spreads the impact slightly to not mark or dent the thing you're hitting (as badly)

And stops rebound.

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