Theory Newton's Cradle

Most readers will likely be familiar with Newton’s Cradle (see wikipedia); if not by name, surely when described as the small rack of steel ball bearings, suspended from wires, that one might find on the desk of an executive. When one ball is set in motion it contacts the row of balls, sending the furthest ball in an equal and opposite motion. The principle behind Newton’s Cradle will act as a fitting example with which to describe the theory and practice of several forms of lock picking techniques that rely on kinetic energy.

In the Newton’s Cradle example, the collision of one ball with a row of balls produces an interesting result. Rather than the balls scattering upon impact, as one might imagine would happen to a group of bowling pins when met with the fate of a ten pound ball, all but the one ball on the furthest end from the contacting ball will remain perfectly still. The one effected ball on the end will be propelled away from the stack at a rate of speed roughly equivalent to that with which the first ball traveled. This is not at all unlike what happens when two adjacent pool balls are struck with a cue ball. What happens is that, upon impact, the kinetic energy (energy of motion) from one ball is transferred though the contacted ball, to the next ball, and so on, until the final ball has no solid object through which to transfer its energy, causing it to propel away from the group. If this sounds at all confusing, you can think of examples of this characteristic at play in both pool and croquette.

This physical principle can be utilized to exploit many common pin tumbler locks. Consider a single pin stack. There exists in the stack (barring any master keying) one spring, one driver pin, and one key pin. The key pin and driver pin sit in contact with each other. This means that, using the principle displayed in Newton’s Cradle, if we strike the bottom of the key pin the kinetic energy of our strike will be transferred through the key pin to the driver pin, causing the driver pin to leap away from the key pin with a rate of force roughly equal to that which we exerted with our strike. For this to work, however, the amount of force we exert in our strike must be sufficient to overcome the downward force of the spring.

Now imagine for a moment this pin stack at rest in its normal state within the chamber of the lock. The driver pin would be crossing the shear line. If we were to strike the bottom of the key pin with enough force to overcome the spring tension, the driver pin would leap away from the key pin, clearing the shear line. If there were only one pin stack in the lock at that moment, and we applied tension at the precise moment that the driver pin was suspended over the shear line, the plug would be free to rotate and the lock would open.

Now imagine, if you will, that the lock was pinned with all five pin stacks. If we could strike all five key pins at the same time, the energy from each key pin would be transferred to its corresponding driver pin, causing each driver pin to leap above the shear line simultaneously. If we were to apply tension at the precise moment that all driver pins had cleared the shear line the plug would rotate, having nothing to block its rotation. This is precisely the principle upon which mechanical pick guns, snap picks, and electric picks is based.

Each of these devices has a thin pick or needle that snaps up rapidly, either by mechanical mechanism or electric motor. When the needle makes contact with the key pins the kinetic energy is transferred through the key pins to the driver pins causing them to hop. These devices, and their use, will be discussed in detail in the following three chapters.

Newton's cradle examples with unequal masses

If the masses in Newton's cradle are not all equal, the situation is more complex, but to avoid considering the physics in detail, we consider two simple examples. Real locks do not in general have all equal masses!

Example 1

Consider three masses 2m, m, and 2m, in that order, and suppose that the first mass is moving at velocity v and the other two are touching and at rest. After the first mass hits the two touching masses, the first and second masses are at rest and the third mass is moving at velocity v. So, in this case, it is almost like Newton's cradle with all equal masses.

Example 2

We use the same three masses as in Example 1, but in a different order. Consider three masses of 2m, 2m, and m, in that order, and suppose that the first mass is moving at velocity v and the other two are touching and at rest. After the first mass hits the two touching masses, the first mass is at rest, the second mass is moving at velocity v/3 and the third mass is moving at velocity 4v/3. Notice here that both the second and third masses are moving in the same direction, and think about the effect this might have in a lock being picked as described above.


However, even when the masses are not equal, we do in general create gaps between the masses that were originally touching, so the hope is that we can arrange it so that this gap contains the shear line for all the pin stacks at the same time. If so, the lock is picked!

Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-Share Alike 2.5 License.