# B Driving a nail into a wooden block

1. Sep 19, 2016

### Amru123

We usually observe that in order to force a nail into a block we have to provide some support to the block.Can someone scientifically explain why we do it?

2. Sep 19, 2016

### phinds

I'm not clear on why you say "usually". What would happen if you sat a 6" long chunk of 2x4 on a work bench and tried to drive a nail into it sideways? What would happen if you tried to drive the nail straight down? Do you understand Newton's Laws?

3. Sep 20, 2016

### CWatters

You don't always have to support the block....

The situation is more complicated than this but on a simple level.. the nail must move faster than the block in order to penetrate it. The problem is that the nail also apply a force on the block that tries to accelerate the block.

Ask yourself what property of the block might help stop it being accelerated or at least reduce the rate at which it accelerates?

What can you do to the block to stop it accelerating/moving if that property isn't sufficient.

4. Sep 20, 2016

### houlahound

You can certainly drive a nail through an unsupported anything.

5. Sep 29, 2016

### jbriggs444

On the other hand, any carpenter knows that it's easier to pound a nail into a pair of boards supported by a third board end-on to the first two than into two boards with no support at all.

When you are hammering a nail into a board, there are a number of things that happen.

1. The hammer strikes the nail (a partially elastic collision). This imparts kinetic energy into the nail and accelerates the nail toward/into the target. However, the nail is of very low mass relative to the hammer. It gains very little kinetic energy and penetrates little, if at all into the target before coming to a stop. The hammer, on the other hand, loses little kinetic energy and keeps on moving.

2. The hammer strikes the nail again and again in a series of collisions that damp out rapidly so that the effect is as if the initial collision had been completely inelastic. Both hammer and nail are now moving at the same speed toward the target. Their joint speed is only slightly lower than the speed with which the hammer first struck.

3a. If the target is held immobile, the business end of the nail is penetrating into the work as the hammer+nail slow to a stop. The force of friction between nail and work is acting to slow the hammer+nail as the penetration proceeds. There is also crushing and deformation at the nail point which also serves to slow the hammer+nail.

3b. If the target is allowed to move out of the way, energy is being dissipated in motion of the target rather than in penetration in the nail hole.

4. Ultimately the hammer+nail come to a stop. Their joint kinetic energy having been expended in the target, one way or the other. The hammer is then drawn back for another blow.

An end-on support will "give" very little under a hammer blow. The energy of the blow is dissipated where it needs to be -- in driving the nail into the work.

6. Sep 29, 2016

### phinds

The OP seems to no longer care. Ten days and no response.