# The physics of dead blow hammers **URL:** \ **Category:** Factual Questions\ **Created:** [May 21, 2020, 3:14pm UTC](https://boards.straightdope.com/t/the-physics-of-dead-blow-hammers/854246 "2020-05-21T15:14:55Z")\ **Posts on this page:** 4\ **Page:** 2
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [May 22, 2020, 12:22am UTC](https://boards.straightdope.com/t/the-physics-of-dead-blow-hammers/854246/21 "2020-05-22T00:22:39Z")
> [@bump](#): > > Could be something about the way wood splits- maybe there’s a component of force over time there that we might not see in other situations. Yes indeed there is. (Per my rudimentary understanding) The impact force from a rigid hammer is like a like a spike over a few milli-seconds. From a softer hammer, the impact force is less but spread over a time. But what is more interesting is if you take a Fourier transform of the force (acceleration), you see that the rigid hammer produces a flat response over all frequencies But the softer hammer has the peak around a lower frequency and it decays for higher frequency. Here is an article on soft versus hard hammers and their frequency response : [http://www2.me.rochester.edu/courses/ME443/Assignments/VibrationTesting.pdf](http://www2.me.rochester.edu/courses/ME443/Assignments/VibrationTesting.pdf) So for the wedge example : maybe it starts resonating at a higher frequency and is pushed out. Chronos also pointed out the heat in soft media. This is a real problem when they are driving piles into the earth or ocean floor (essentially a big hammer) And they have a soft cushioning material. I have incomplete knowledge gleamed from civil engineers doing the pile designs. I think a civil engineer will probably give you a better answer. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [May 22, 2020, 12:45am UTC](https://boards.straightdope.com/t/the-physics-of-dead-blow-hammers/854246/22 "2020-05-22T00:45:41Z")
> [@Andy\_L](#): > > When someone swings a dead-blow hammer, do they swing harder, because they know that there will be no dangerous rebound? (P.S. This is a sincere question - I’m just wondering). I don’t think so; it certainly isn’t true in my case of splitting wood because I find I can actually get a deeper split with less developed force using a dead-blow hammer, whereas with the solid sledge I’m usually hitting it with everything I have to ensure the wedge doesn’t pop out. I did start using a dead-blow because when I’m using that and a wedge or friend, I’m often trying to free a maul that has gotten stuck in a knotty or resinous piece of wood by splitting it on the opposite side and I’m concerned about the maul springing out and flailing in a random direction. With a dead-blow, even if the first hit doesn’t split the round the maul often just falls out or becomes loose enough to pull out with little force. Stranger ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [May 22, 2020, 1:08am UTC](https://boards.straightdope.com/t/the-physics-of-dead-blow-hammers/854246/23 "2020-05-22T01:08:56Z")
> [@am77494](#): > > But what is more interesting is if you take a Fourier transform of the force (acceleration), you see that the rigid hammer produces a flat response over all frequencies But the softer hammer has the peak around a lower frequency and it decays for higher frequency. > > Here is an article on soft versus hard hammers and their frequency response : [http://www2.me.rochester.edu/courses/ME443/Assignments/VibrationTesting.pdf](http://www2.me.rochester.edu/courses/ME443/Assignments/VibrationTesting.pdf) > > So for the wedge example : maybe it starts resonating at a higher frequency and is pushed out. Since this is a shock event you actually want to calculate the [shock response spectrum (SRS)](http://www.sandv.com/downloads/0906alex.pdf) or the [pseudovelocity shock spectrum (PVSS)](http://sandv.com/downloads/1201gabe.pdf) (you can actually calculate one from the other, they’re just different ways of visualizing and interpreting the shock response). You are essentially correct that as measured on the hammer a solid head hammer is going to have a broad plateau on the PVSS across a wide frequency band where a soft/shot-filled head will attenuate dramatically at medium and high frequencies because of damping. Although in basic physics and mechanics texts an impulse is often characterized as a single impulse and resulting equivalent force, in reality every mechanical impulse or vibration into a solid elastic medium results in an oscillatory response where the vibrations tend to drive the resonances of the system, and a sharp shock essentially excites all resonances simultaneously, so you get amplification of those resonant frequencies. In the case of a steel-on-steel hammer, the response is an acoustic (or pressure) wave within the wedge that returns back to the striking hammer very quickly (speed of sound in steel is almost 6000 m/s) with most of the input energy kicking it away or transmitting into the head, whereas with a dead-blow hammer the low frequency energy spectrum is just absorbed by the dead-blow and the high frequency bounces back through the wedge repeatedly because it will have low transmissibility to the head. Stranger ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [May 22, 2020, 1:25am UTC](https://boards.straightdope.com/t/the-physics-of-dead-blow-hammers/854246/24 "2020-05-22T01:25:39Z")
> [@Stranger\_On\_A\_Train](#): > > I don’t think so; it certainly isn’t true in my case of splitting wood because I find I can actually get a deeper split with less developed force using a dead-blow hammer, whereas with the solid sledge I’m usually hitting it with everything I have to ensure the wedge doesn’t pop out. I did start using a dead-blow because when I’m using that and a wedge or friend, I’m often trying to free a maul that has gotten stuck in a knotty or resinous piece of wood by splitting it on the opposite side and I’m concerned about the maul springing out and flailing in a random direction. With a dead-blow, even if the first hit doesn’t split the round the maul often just falls out or becomes loose enough to pull out with little force. > > Stranger Thanks [Previous page](https://boards.straightdope.com/t/the-physics-of-dead-blow-hammers/854246.md?page=1)