# Metal hammer vs rubber mallet

Simply put, which one works better?
Let’s suppose I have a rubber mallet and a metal hammer, essentially identical in every aspect other than the stuff they’re made of. As I see it, when the metallic one comes in contact with the nail, the collision can be modelled as a perfectly inelastic one, so all of the hammer’s momentum and kinetic energy would be transferred to the nail. On the other hand, the rubber mallet hits the nail, it’s a perfectly elastic collision we’re talking about, since the mallet recoils at about the same speed as it had when it hit the nail; so twice its momentum, but no kinetic energy would be transferred to the nail?!?

I guess what you’re getting at must really be a physics question, because they’re not identical in any aspect, and you don’t use a rubber mallet to drive a nail.

The model you describe for the rubber mallet only works if the nail is in an impenetrable substance. The mallet would bounce off and some energy would be transferred to the nail since the nail did exeprience a force, and that energy would take the form of heat. Other energy would take the form of heat in the mallet from the deformation and reformation. If the surface were ordinary wood then the nail would drive into the wood to some degree, even with an elastic collision with the mallet, which would also result in some heat.

BTW steel is rather elastic. I guess when you say “inelastic” you are talking about the permanent deformation of the nail and the wood as a system, i.e., the nail goes into the wood.

All of this notwithstanding I’m not sure what you’re question is. Which one works better for driving a nail? The metal hammer. But your analysis shows you knew that already.

Still, the momentum transferred from the rubber mallet is greater than that from the metal hammer (let’s suppose they have the same mass), for the recoil is greater. Why is the metal hammer more effective, then?

The steel hammer will be more elastic than the rubber one, and therefore impart more momentum to the nail. Steel is much more elastic than rubber, in the physics sense of the term, and elastic collisions transfer more momentum.

And steel hammers are more effective at driving nails, but rubber ones are more effective at (for instance) driving chisels. They’re intended for completely different purposes.

Also, it can make a difference what you’re hitting. If you’re driving in metal tent stakes, you need to use a metal hammer or you’re just going to chew the heck out of a rubber mallet. On the other hand, if you try to pound in plastic stakes with a metal hammer, you stand a much better chance of breaking the stake.

But less kinetic energy?
Also, I’ve seen the rubber mallet recoil more than the steel hammer. How is it that steel is more elastic?

And what’s the physical rationale behind that?

Let’s suppose, just as in the physics textbooks, both hammer and object being driven are unbreakable…

Here’s my guess as to what’s going on:

So suppose that you have a rubber mallet and a steel mallet about to impact on a nail, both having the same kinetic energy. They will both contact the mallet and begin to deform, the nail doing work on each mallet as it comes to a rest. However, the rubber mallet will deform more (i.e. a greater distance) than the steel mallet. Since the same amount of work was done on each mallet (they had the same initial and final kinetic energies), since work is distance times force, and since the work done on the steel mallet was done over a shorter distance, we conclude that the steel mallet exerted a greater force on the nail.

Aha, you say, then why did the nail hit by the steel mallet go in further, when the force wasn’t exerted over very much distance? Well, the truth is that the above analysis only really works while we’re trying to get the nail started, and it’s not yet moving. When you’re driving a nail into wood, you’re essentially trying to make the wood grains “fail” in some way. Once you’ve managed to make part of a structure fail (in this case, part of the wood grain), it’s often not too hard to make the rest of the structure fail as well. We all saw a particularly ghastly example of that about five years ago.

So the basic reason you want to exert a steel mallet rather than a rubber one is that it exerts much more force on the nail, and this force is enough to drive the wood into “failure mode” while that exerted by a rubber mallet isn’t.

Hope this helps…

I don’t think that assumption is valid. Since the rubber mallet bounces more after hitting the nail than the steel mallet, wouldn’t that mean that its final kinetic energy is greater than that of the metallic one?

Metal hammer vs rubber mallet

Which one works better for what?
For practical uses…
Metal hammers are for driving nails, heavy duty staples, etc.
A rubber mallet is for use in fitting and assembly of wooden components.
A heavy mallet with rawhide faces for assembly of metal components.
And there at times that a small metal mallet with plastic or brass faces is handy too!
PS There is also a ‘dead’ hammer or mallet that doesn’t bounce at all.

I think this is incorrect; the recoil is greater because rather than being transferred to the nail, the energy is temporarily stored as potential energy in the elastic deformation of the rubber; this potential energy is released, acting to cause the recoil.

If more momentum was transferred, it would recoil less.

Nope.

Before hitting the nails, both hammers have momentum p.
After hitting the nail, the metal hammer has momentum 0 (let’s suppose it doesn’t recoil at all), so the transferred momentum is p-0=p.
Likewise, the rubber mallet has momentum -p after hitting the nail. Therefore, the transferred momentum is p-(-p)=2p.

Which one is obviously greater?

If the nail was a theoretically immovable object, would any momentum be transferred?

What if Jesus was swinging the hammer?

Wait a minute; so the hammer is transferring more momentum than it actually has? There should be a law against it.

Who said the nail was a theoretically unmovable object?

More momentum than it has with respect to you, yes, and much more than it has with respect to itself. But not more than it has with respect to, say, a mosquito which is flying away from the nail when it sees the hammer.

As a machinist I use a dead blow hammer with one good thwack to set a workpiece in a vise before milling it. Using a steel hammer makes the workpiece bounce back against the bottom of the vise and it is loose in the vise.

I don’t want to hurt my head with the math and or physics but in a real world application there is a reason for a dead blow mallet.

I mean from the point just before the mallet contacts the hammer to the point where it comes to rest. Sorry if that wasn’t clear.

Does it actually come to rest? If the hammer bounces back, it would continue moving indefinitely (in the absence of air resistance, opposite walls, or bystanders which might be hit by the hammer) and not come to rest.

Or am I missing something?

<CHL>What we have here is failure to communicate.</CHL>

The mallet comes in and hits the nail. As the material of the mallet deforms, it slows down. Eventually, it stops. Then the head of the mallet begins to restore to its shape, and eventually rebounds. It’s the intermediate stopping point, where the mallet has reached its maximum deformation, that I’m referring to.