h a l f b a k e r y
On the one hand, true. On the other hand, bollocks.
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Carpenter's hammer with movable weight. Spring and back
swing keep weight away from main head until split second
after main head hits the nail. Movable weight is counter
bored so the spring, when compressed, is inside it and weight
hits the main head. Total weight and weight distribution are
the same as standard hammer. I know you don't get
something for nothing from Ma Nature so MFD away!
[cudgel, Sep 18 2015]
Low rebound hammer filled wih lead shot [csea, Sep 18 2015]
Elastic vs Inelastic Collisions
Rebound is better. [MechE, Sep 28 2015]
||like a dead-shot hammer with a solid lump not lead balls.
You'd have to figure out how not to have problems with the
bounce. You could just double the mass of the hammer for
the same effect?
||Make it a triple strike hammer. Or, dare I suggest, a quad
strike hammer. Or, let's get real crazy...an infinite strike
hammer. Stick a jackhammer mechanism in the hammer
head, with a large mass to absorb the recoils.
||The bounce of the striking surface is a (surmountable)
challenge. The main head would need low mass and very
high stiffness. After hitting the nail it actually bounces off
and is stopped...it has to be light so the heavy weight can
make it strike again.
||I'm with [bs]. If the system works perfectly, you will
deliver the same total energy as with a normal
hammer of the same mass, but as two small taps
rather than one big one. I'm not seeing an
advantage; in fact, the opposite.
||It actually doesn't rebound even hitting a steel table at an
easy swing. The spring seems to have something to do with
it. The two strikes come so close together I can't really hear
||You made this? Well, kudos for that.
||[MB] When I had this idea I expected it would deliver
lower performance than a regular hammer of the same
weight. I was very careful to make it as alike as possible
(within .2 oz.) of the same conventional hammer. Not
expecting an advantage but a disadvantage, therefore
Half Baked. The dead blow effect was unexpected. It is
hard for me to tell a difference when driving 12d nails
with either hammer into the same piece of wood. The
spring started out on the back side of the movable head
with the question being would it be compressed enough
by the backswing to provide a little extra tap? The double
strike sound was just a bit noticeable in that
||There are times when multiple lighter blows are
desirable, but they are rare. In blacksmithing lighter
blows work more on the surface of the piece whereas
harder blows have an effect through the piece, and
ocassionally this is useful if you are trying to do certain
types of detail work. However, it is usually simpler just
to switch to a lighter hammer.
||Because, a deadblow hammer (no matter how it is
accomplished) is less efficient than a hammer that
rebounds. With a traditional soft face mallet, the extra
energy goes into deforming the face. With this, I suspect
it goes into compressing the spring, but doing so
insufficiently to produce a recoil.
||So it's interesting, but probably useless.
||I guess any rebound of the first mass is suppressed
by the following mass.
||Hmm. There's an interesting physics question
here. Suppose you have two hammers of identical
mass, swung at the same speed to strike a nail.
One hammer has some rebound, the second
hammer has no rebound. Which is more effective
at driving the nail?
||1st argument: the rebounding hammer leaves with
some kinetic energy after the impact; therefore it
must deliver less kinetic energy to the nail, and
will be the less effective.
||2nd argument: the reboundless hammer has all of
its downward momentum absorbed by the nail.
However, the rebounding hammer not only loses its
downward momentum, but acquires some upward
momentum on the rebound. This means that the
force between the nail and the rebounding
hammer must be higher, so the rebounding
hammer must be the more efficient.
||[MechE] Yep, just a wall hanger.[MB] The physics is way
beyond me but interesting that things don't always fail or
succeed the way you think they will. If you can see the
photos you know that the face that hits the nail is
connected to much more mass than the movable head
but less than a normal hammer. I've also wondered if the
double, or multiple strikes, if built into the head of a golf
driver would be any advantage. Would the club face and
ball stay in contact a wee bit longer to transfer more of
the energy of the swing to the ball? The downswing of a
driver is much faster than that of a hammer so would
compress stronger(relatively) springs. Physicists please
||[MBhammer] I think your system is too small, hence the conundrum.
The mass of the nail and the presumed board , and the stick /slip
frictional profile of the nail vs board should be accounted for.
||Kind of like the multi-blade razor situation, or Newton's balls. Max.
energy transfer in an elastic collision occurs with identical masses.
||//The mass of the nail and the presumed board ,
and the stick /slip frictional profile of the nail vs
board should be accounted for.//
||OK, so let's deal with those. Assume that the nail
has negligible mass (compared to the hammer
head). Assume also that we are driving the nail
into an extremely viscous ideal liquid; that
overcomes the complexities of "static" versus
||So now which hammer displaces the nail furthest:
the dead-weight hammer that has no kinetic
energy after the impact; or the rebounding
hammer whose momentum is not only absorbed
||So is the compression of the steel in the nail and the
hammer head nullified as to rebound effect in one and not
||Well, in one case that compression is causing the
head to rebound, and in the other case it isn't...
||It would seem that the compression and rebound of the
steel contributes to driving the nail as well as rebound of
the normal hammer. Is this lost with no rebound of the
||I think the rebounding hammer gets its energy split by the
friction of the nail, kind of like two pool balls being hit at
the same time by a perpendicular strike from the cue
ball. Half goes one way, half goes the other. A strike from
the cue ball to both of those balls in line gets all the
energy put into it going to one place, wherever both balls
hit which would be analogous to the bounce free
||I'm wondering though if any "shock absorbing" mechanism
you put into the hammer to nullify the rebound looses
energy in the form of heat or something.
||But definitely, a car crashing into a mud wall vs one
bouncing off a brick wall, the mud wall of the same mass
is getting pushed further back because it's holding onto
the car through the full dissipation of its energy, from the
time it hits till the time it comes to a full stop.
||I think the inertia makes the difference. The moving car
(or hammer) only has split second to overcome the
inertia of the brick wall, then its energy goes off the
other way. If there's something to hold onto that moving
object for a longer time, the inertia is overcome more
effectively. Like running as fast as you can into a heavy
refrigerator to move it vs just pushing it.
||I wonder if any extra advantage is lost in the energy taken to swing it?
||//So now which hammer displaces the nail furthest: the dead-weight
hammer that has no kinetic energy after the impact; or the rebounding
hammer whose momentum is not only absorbed but reversed?//
||Quite clearly the dead weight example transferrs the most energy to
moving the nail P=mv. The rebounded hammer takes away a portion
of the original energy and shows it up as kinetic, then gravitational
energy, some of which may be used to pound the nail again.
||What hasn't been addressed is oops, looks like [2fries] beat me to it...
||// the dead weight example transferrs the most
energy to moving the nail P=mv//
||Uh, the energy would be 1/2x mv^2.
||The momentum transferred would be mv; but the
rebounding hammer must transfer more momentum
to the nail, since the velocity is negative on the
rebound. Hence the conundrum.
||I think the rebounding hammer pushes the nail the
||The dead-weight hammer scenario is an inelastic collision.
I.e. some of the initial energy of the system is wasted as
heat generated during the inelastic compression of the
materials. In the case of the rebounding hammer, a portion
of this energy is recovered as kinetic energy, both of the
hammer and nail.
||So in other words, I think you want the hammer to bounce
as far as possible, and then use this recovered energy to
further drive the nail via multiple strikes.
||And I've just realised this is basically what [MechE] said.
||Actually, it might be better to use a screw.
||I see it this way, in perfect envelope ideals. Action x drives a nail into a material to it's maximum distance. A non rebounding hammer with an action x swing, drives it to this distance = total transfer . A rebounding hammer with action x swing drives to maximum minus completely opposedly wanted rebound action. In both, real cases the hammers would have other losses.
||A quick double strike might be an advantage in materials that close up fast and you want to gain on the first hit.
||The problem is that your boundary conditions are
an impossible case, so they don't work. It is not
possible to have an inelastic collision where kinetic
energy is conserved. If you hit something with a
hammer that doesn't rebound, a (largish) chunk of
the energy is going somewhere other than the nail,
typically to (permanently) deforming the head, or
||A rebounding hammer, on the other hand,
conserves both momentum and kinetic energy.
And since the hammer's momentum is now
negative, the nail has a much greater forward
||To clarify, picture two objects of equal mass, m, in
free space. One stationary and one with velocity,
||In an inelastic collision (non-rebounding), they
start with kinetic energy mVi^2, and momentum
mVi. When they collide, they stick together, and
now have momentum 2mVi/2, or still mVi (twice
the mass, half the speed, since momentum is
conserved). However, their kinetic energy is now
2m(Vi/2)^2, or mVi^2/2, half of what it was initially.
This is because kinetic
energy isn't conserved, and the remainder is lost as
heat. The more massive the second object, the
more energy is lost to heat (and a nail with a wall
behind it is fairly massive).
||In the case of a perfectly elastic collision, the ball
with the initial velocity is left stationary, and the
ball 2 is now moving away at the same velocity the
original ball was. The equation for velocity of the
first mass in an elastic collision is V1f=V1i*(m1-
m2)/(m1+m2). (This may seem counterintuitive,
but this is why it's possible for the cue ball to stop
dead after hitting another pool ball). Thus all of
the kinetic energy is transferred to the second
||If the second ball is more massive, it's final speed
will be slower, V2F=V1i*2m1/(m1+m2), but the
resultant kinetic energy sill still be higher than the
inelastic collision mentioned first.
||Why is the spring necessary? As long as you're bringing the
hammer down at faster than the speed of gravity, inertia
should keep the weight separated from the back of the head
until the strike.
||[MechE] The two cases have the same masses, the hammers and the (nails forced into material), the same action energy, the swing. One hammer leaves with energy, one doesn't. I would be surprised if the energy from the heat and sound >= the E from the rebound.
||The spring would be good for other hammer motion rather than have a weight flopping around.
||[ytk] Yes, the spring isn't necessary. My original idea placed
the spring behind the floating weight.
||[wjt] Check the math. Momentum is conserved, so no
matter what, the kinetic energy of the nail then the
hammer comes to a dead
stop is 1/2 the kinetic energy of the nail when the hammer
rebounds. The energy lost in an inelastic
absolutely greater than the rebounding energy.