Cards | 10 |

Topics | Block and Tackle, Building Loads, Conservation of Mechanical Energy, Joules, Kinetic vs. Static Friction, Modulus of Elasticity, Power, Specific Gravity, Structural Loads |

Two or more pulleys used together constitute a block and tackle which, unlike a fixed pulley, does impart mechanical advantage as a function of the **number of pulleys** that make up the arrangement. So, for example, a block and tackle with three pulleys would have a mechanical advantage of three.

**Dead load** is the weight of the building and materials, **live load** is additional weight due to occupancy or use, **snow load** is the weight of accumulated snow on a structure and **wind load** is the force of wind pressures against structure surfaces.

As an object falls, its potential energy is converted into kinetic energy. The principle of conservation of mechanical energy states that, as long as no other forces are applied, total mechanical energy (PE + KE) of the object will remain constant at all points in its descent.

The Joule (J) is the standard unit of energy and has the unit \({kg \times m^2} \over s^2\).

For any given surface, the coefficient of static friction is higher than the coefficient of kinetic friction. More force is required to initally get an object moving than is required to keep it moving. Additionally, static friction only arises in response to an attempt to move an object (overcome the normal force between it and the surface).

The modulus of elasticity measures how much a material or structure will deflect under stress. **Stretch modulus** is longitudinal stretch (like stretching raw bread dough), **shear modulus** is longitudinal deflection (like the horizontal displacement of a stack of magzines when a heavy object is placed upon them), and **bulk modulus** is compression of volume (like the compression of a loaf of bread under a heavy can at the bottom of a grocery bag).

Power is the rate at which work is done, **P = w/t**, or work per unit time. The **watt (W)** is the unit for power and is equal to 1 joule (or newton-meter) per second. **Horsepower (hp)** is another familiar unit of power used primarily for rating internal combustion engines. A 1 hp machine does 550 ft⋅lb of work in 1 second and 1 hp equals 746 watts.

Specific gravity is the ratio of the density of equal volumes of a substance and water and is measured by a hyrdometer.

A **concentrated** **load** acts on a relatively small area of a structure, a **static uniformly distributed load** doesn't create specific stress points or vary with time, a **dynamic load** varies with time or affects a structure that experiences a high degree of movement, an **impact load** is sudden and for a relatively short duration and a **non-uniformly distributed load** creates different stresses at different locations on a structure.