47) LIST OF ENGINEERING TOPICS FOR LETTER "U"

Uncertainty principle
In quantum physics, the Heisenberg uncertainty principle states that certain pairs of physical properties, like position and momentum, cannot both be known to arbitrary precision. That is, the more precisely one property is known, the less precisely the other can be known. It is impossible to measure simultaneously both position and velocity of a microscopic particle with any degree of accuracy or certainty. This is not a statement about the limitations of a researcher's ability to measure particular quantities of a system, but rather about the nature of the system itself and hence it expresses a property of the universe.

In quantum mechanics, a particle is described by a wave. The position is where the wave is concentrated and the momentum is the wavelength. The position is uncertain to the degree that the wave is spread out, and the momentum is uncertain to the degree that the wavelength is ill-defined.

The only kind of wave with a definite position is concentrated at one point, and such a wave has an indefinite wavelength. Conversely, the only kind of wave with a definite wavelength is an infinite regular periodic oscillation over all space, which has no definite position. So in quantum mechanics, there are no states that describe a particle with both a definite position and a definite momentum. The more precise the position, the less precise the momentum.

The uncertainty principle can be restated in terms of measurements, which involves collapse of the wavefunction. When the position is measured, the wavefunction collapses to a narrow bump near the measured value, and the momentum wavefunction becomes spread out. The particle's momentum is left uncertain by an amount inversely proportional to the accuracy of the position measurement. The amount of left-over uncertainty can never be reduced below the limit set by the uncertainty principle, no matter what the measurement process.


Unreinforced masonry building
Unreinforced masonry building (or UMB) is a type of building where load bearing walls, non-load bearing walls, or other structures such as chimneys are made of brick, cinderblock, tiles, adobe, or other masonry material that is not braced by reinforcing beams. The term is used as a classification of certain structures for earthquake safety purposes, and is subject to some variation from place to place.

UMB structures are vulnerable to collapse in an earthquake. One problem is that most mortar used to hold bricks together is not strong enough.Additionally, masonry elements may "peel" from the building and fall onto occupants or passersby outside.

In California, construction of new unreinforced masonry buildings was prohibited in 1933, and state law enacted in 1986 required seismic retrofitting of existing structures. Retrofits are relatively expensive, and may include tying the building to its foundation, tying building elements (such as roof and walls) to each other so that the building moves as a single unit rather than creating internal shear during an earthquake, attaching walls more securely to underlying supports so that they do not buckle and collapse, and bracing or removing parapets and other unsecured decorative elements. Retrofits are generally intended to prevent injury and death to people, not to protect the building itself.

The California law left implementation, and standards, up to local jurisdictions. Compliance took many years. As of 2008 most but not all unreinforced masonry buildings have undergone some retrofitting.