**Length **- Linear or curvilinear distance between two points. Linear distance is the length along a straight line. For example,determine the length of a string that can stretch between two points on your classroom floor. Curvilinear distance can be the distance around a circle or other curved shape. For example, determine the length of a string (also a line) that can stretch around your wrist. Typical units for length are inches, feet, meters, centimeters, etc.

**Area** - The two-dimensional size of a surface is its area. Expressed in square units such as square inches, square feet, square meters, etc. Think of it, for example, as how many square inches can you paste onto the surface of something (i.e. the floor of your classroom or a basket ball). The surface can be flat or curved.

**Volume** - The three-dimensional measure of the space occupied by an object or substance. Expressed in cubic units such as cubic inches, cubic feet, cubic meters, etc. For example, how much volume does your classroom occupy? Or, how many cubic feet will fit into you classroom.

**Force** - Any action applied to an object which would cause the object to move, change the way it is currently moving, or change its shape. A force can also be thought of as a push (compressive force) or pull (tensile force) acting on an object. For example, you might push or pull a desk to move it. You might want to exert a force on a soccer ball to prevent it from going into your goal. Some typical units are pounds, newtons, tons, kips (kilo-pounds 1 kip = 1000 lbs), etc.

**Force Vector** - A force vector is a graphical representation of a force. Force vectors are drawn as arrows and show the direction of the force and its point of application. Let's assume that you want to draw a sketch of a desk and show a force vector that represents someone pushing, with a force of 15 pounds, on the desk. You could draw a rectangle for the desk then draw an arrow for the force vector. Place the head of the arrow at the point where the person is touching the desk and align the stem of the arrow in the direction of the push.

**Pressure **- An external force applied over some area. Expressed in force per area unit (P=F/A). For example, let's say there is a one foot cube that weighs 200 pounds sitting on the driveway at your house. How much pressure is the box exerting on your driveway? P=F/A Therefore, Pressure = 200 pounds divided by an area of 1 square foot (one times one - the surface of the box in contact with the ground). The answer is 200 psf (pounds per square foot).

**Stress** - Stress can be thought of as pressure created within an object. For example, if I apply a tensile force (see force definition above) at both ends of a board, how much stress will I create in the board?

**Mass** - This is the amount of matter contained in an object. It's best to think of mass while in deep space - away from the effects of gravity. Pretend you are space walking outside your craft. There is an object nearby that you must move into the cargo bay of your space ship. If the object has little mass like balsa wood, it might be easy to get it to move toward the cargo bay. Let's say there is another object with the same physical size (i.e. occupies the same volume) as the balsa wood. However, this new object is solid steel. It will be much harder to get the steel object to start moving toward the cargo bay. The steel object has more mass.

**Weight** - The force of attraction caused by gravitational pull. We need to think of gravity right here on earth ( or on some other planet). Weight (here on earth) is a force that acts on a body or object and is directed toward the center of the earth. What is your weight? This force that is your weight is directed along a line of action that goes through you and the center of the earth ( really the center of mass of the earth).

**Density** - There are two kinds of density - mass density and weight density. Both types are a measure of how much matter is in a given volume.

Let's consider a block of balsa wood. If you look very closely at the end grain of the wood you will see little holes. This means that the block is not entirely composed of wood matter. There is air occupying the space defined by the holes. Actually there are many tiny holes and gaps that you can't see (without a microscope).

This means that density is also a measure of how porous (filled with holes and gaps) the material is.

Mass density is the mass of the object divided by the volume of space that the object occupies. Weight density is the weight of the object divided by the volume of space that the object occupies.

For example, let's calculate the weight density of a 1/8"x1/8"x24" stick of balsa wood. The volume of the wood is V=(.125)(.125)(24) = .375 cubic inches. Next, you must take the stick of balsa to a scale and weigh it. Let's say it weighs .053 oz. Then the weight density is D=.053/.375= .1413 oz per cubic inch.

If we divide by 16 we get D=.00883 pounds per cubic inch. And if we convert the units to pcf (pounds per cubic foot) we will have our answer in units that are normally used to express the density of wood. D=.00883(12)(12)(12) = 15.3 pcf.

The weight density of balsa wood varies greatly. This stick of balsa that we checked has a density of 15.3 pcf. To compare, the weight density of southern pine is about 36 pcf.

Remember, from the definition of density, the greater the density the greater the matter (wood in this case) within the volume. And, generally, that means there are more wood fibers in the volume available of resist load.

**Loads **- When engineers refers to loads they usually mean the entire collection of forces acting on the object. This would include any moments acting on the object. Sometimes the term load is used to mean only those forces caused by gravity (i.e. weight forces).

**Moment** - A measure of the tendency of a force to cause rotation about an axis. The graphical representation of a moment acting on an object is called a curl. A curl is an arc shaped arrow drawn near and about the axis of rotation. Typical units are in-lbs, ft-lbs and ft-kips, N-m ( some professions use lb-ft and kip-ft, many engineers like the more common ft-lbs and ft-kips - either is correct). Moment(M) = Force(F) times the perpendicular distance to the axis(d). M = F x d.

**Work** - Work is force times distance. The force is that force (or component of force) in the direction of movement times the length of movement. Typical units are ft-lbs, ft-kips, N-m and in-lbs. For example, if you raise a weight of 75 pounds from a height of 3 feet to a height of 5 feet, you will have done (75)(2)=150 ft-lbs of work. Work can also be defined as moment times the angle of rotation for circular movement.

**Power** - This is amount of work done divided by the time it took to do the work. A 10 hp (horsepower motor can raise a 75 lb weight twice as fast as a 5 hp motor. The 10 hp motor is more powerful. One hp equals 746 watts or 550 ft-lbs per second.