So, what I’m wondering is if the color blue is a characteristic favorite color for engineers (and prospective engineers). After doing some research on the internet, here’s what I came up with about the color blue and what it means if you consider it to be your favorite color:

• Squidoo says “Blue is the overwhelming ‘favorite color.’ Blue is seen as trustworthy, dependable and committed. The color of sky and the ocean, blue is perceived as a constant in our lives.”
• Care2 says that the color blue is “soft, soothing, compassionate and caring, Blue is the color of deliberation and introspection, conservatism and duty. Patient, persevering, conscientious, sensitive and self-controlled, Blues like to be admired for their steady character and wisdom. They are faithful, but are often worriers with somewhat inflexible beliefs and can be too cautious, and suspicious of flamboyant behavior.”

Do you think that a person’s favorite color says something about his or her personality, and if so, does it make sense that many engineers say blue is their favorite color?

Pouring Concrete Over Rebar in Washington State

This is a simple one, though I’m not sure why some people would confuse the two. Sure both are used in construction, and both are related building materials, but they aren’t the same and definitely shouldn’t be used interchangeably.

The rule of thumb is, if the choice is between concrete and cement, the structure in question is probably made of concrete. Generally, concrete comprises of cement, or cement is used to make concrete. (What you buy in home improvement stores is cement, not concrete!)

Cement is the dry powdery material that mixes with water and other additives to make concrete, though cement is also used to make materials like mortar, plaster, and stucco. The most common variety of cement is known as Portland cement, whose name is derivative of the natural stones from Isles of Portland in England. In 1824, a patent was issued for the manufacturing process of what would become known as Portland cement, which is generally gray or white. Cement is primarily made up of calcium and silicon from limestone, and smaller amounts of aluminum and iron from iron ore. The raw materials are mixed in proper proportions and then crushed and grounded to a powder. The powder is sintered in a kiln and clinker is produced. (Sintering is the process of fusing individual masses by heating rather than melting.) Clinker combined with small amounts of gypsum gives us cement.

Once we have cement, concrete is made by adding water and coarse aggregate, like granite, limestone, old concrete, or sand. Through the process of hydration (chemical reaction involving water), cement reacts and hardens when dry. (Follow proper instructions on the cement bag.) Concrete is the most common building material in the world, used to make roads, foundations, walls, bridges, etc.

As a material, concrete is extremely strong in compression (about 4000 psi or 28 MPa), but it is used at the expense of size and weight. It is important to remember, though, that concrete is weak in tension. If concrete is being pulled apart, the cement used to hold the aggregate together is stressed, and the structure is prone to cracking. Similarly, bending is an inherently big problem for concrete structures. Well, to counter this problem, we came up with the idea of reinforced concrete, which uses materials strong in tension to take the tensile stress that may develop. Often, steel bars, which are called rebar (reinforcement bar) are used. Grooves on the rebar help “stick” to the concrete to maximize effectiveness; slipage would defeat the purpose of the rebar.

Also, I found out why rebar is usually green. Apparently, rebar is coated with green epoxy to protect it from rust and corrosion. It’s funny how everything has an explanation in engineering; everything has a purpose.

Hopefully after reading this, there will be no more confusion between cement and concrete. As always, if you have any questions or comments, leave one below!

Sources:
http://www.buildeazy.com/newplans/eazylist/cement.html
http://en.wikipedia.org/wiki/Cement
http://en.wikipedia.org/wiki/Concrete

F = ma

So, what is all of this? Well, let’s start with what we know. Mass can loosely describe how much “stuff” something contains. It can be determined by the product of a material’s density and its volume. We know this as the kilogram, if we follow SI units. Remember that mass is intrinsic of a specific object, and it does not change if we weigh it on the earth or if we weigh it on the moon. Weight, on the other hand, is a function of the gravity (so it’s different on the earth and the moon). Though it is commonly know as “weight”, scientists and engineers know this simply as “force”. Force equals mass times acceleration, with units of “Newton”. (When we talk about weight, we let acceleration simply equal gravity, or 9.81 m/s^2!)

W = mg

In SI units, 1 Newton is the force needed to accelerate a 1 kilogram mass 1 meter per second over 1 second’s time.

When we take a look at English units, though, the equivalent base unit for mass is called the “slug”. By analogy, we have that 1 pound-force accelerates 1 slug mass 1 foot per second over 1 second’s time.

Although “slugs” are the base units of mass for this system, sometimes the “pound-mass” is used to represent a mass. You can take the following to be true: 1 slug = 32.17 lbm. (Both are English units of mass! But remember to use “slugs” in formula that specify a mass. Not pound-masses!)

There we have it. If you remember that slugs are good and pound-masses are “bad” to play around with, everything should work as you’re probably used to with SI units. Use slugs in your equations. Simply as that. Well, kind of.

Here’s the rub, 1 lbf is the same as 1 lbm if the acceleration is equal to gravity, which is 32.17 ft/s^2. Weird! So you can weigh yourself and the scale reads both pound-force and pound-mass at the same time. I guess that is where the usefulness of the pound-mass comes in, but generally, pound mass shouldn’t be used.

One more thing … To confuse things even more, there is yet another set of mass-force pairs that are used. From the discussion above, we now understand that that 1 pound-force accelerates 1 pound-mass 32.17 feet per second over 1 second’s time. To normalize this, we might try to find a force that can accelerate 1 pound-mass 1 foot per second over 1 second’s time. This particular force is called the “poundal”, which is equal to 1/32.17 of 1 pound-force. So,1 poundal accelerates 1 pound-mass 1 foot per second over 1 second’s time (not to say this isn’t useful, but come on!).

Let’s try an example. Say we want to calculate the force exerted on a person, named Bob, driving in his Volkswagen, accelerating at 20 ft/s^2. We weigh Bob and find that his weight is 160 pounds-force (note that this is the weight directly off the bathroom scale, also 160 pounds-mass if gravity at the specific location is 32.17 ft/s^2). To convert his earth weight to a universal mass, we divide by the acceleration of gravity: 160 lbf /32.17 ft/s^2 = 4.97358 slugs. Finally, to find the horizontal force exerted on Bob, we use the traditional F = ma equation: (4.97358 slugs) * (20 ft/s^2) = 99.47 lbf.

There is a really useful table on Wikipedia. It’s copied below, but it is a little confusing to read. I merely distilled it into writing this post.

Mass-Force Unit Systems

Questions? Definitely leave a comment, and I hope we can clarify things for you.