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  • Unit conversion problems come up a lot in science. Sometimes you hear this style of

  • problem solving calledDimensional Analysis.” This is an approach to problem solving that

  • involves using conversion factors and making sure your units cancel. The trick is to look

  • at the units you start with, look at the units you want to end up with, and use a conversion

  • factor to get there. starting unit x conversion factor = ending unit.

  • The conversion factor looks like this: ending unit over starting unit.

  • So see how starting unit and starting unit cancel to give you the ending unit.

  • Sometimes, you may need two or more conversion factors - were going to save that for the next video.

  • A conversion factor is a fraction that equals one. Like, 12 inches over 1 foot is a conversion factor.

  • 12 inches is the same as 1 foot, so 12 inches divided by 1 foot equals 1. So if

  • you multiply your original value by a conversion factor, you don’t change its value - because

  • you are just multiplying by 1 - you just change the units.

  • We could use this conversion factor to go from inches to feet, or from feet to inches.

  • Let’s say I started with 36 inches, and I wanted to know how many feet that was. I

  • know that 1 foot has 12 inches, so I can write that as a conversion factor

  • 1 foot / 12 inches Now I can multiply these together, making

  • sure my units cancel. 36 inches x (1 foot/ 12 inches) = 3 feet

  • Notice that I wrote the conversion factor with feet on top, and inches on the bottom,

  • because I wanted inches to cancel, and I wanted feet in my final answer.

  • What if I wanted to convert in the opposite direction? What if I know something is 5 feet

  • long, but I want to know how many inches that is - I can use that same conversion factor,

  • just flipped. I don’t have to memorize how to set up the problem - I just make sure the

  • units cancel. I start with 5 feet, multiply by the conversion

  • factor so feet is on the bottom, so it will cancel, and inches is on the top so it will

  • be the final units in my answer. 5 feet x (12 inches/ 1 foot) = 60 inches

  • Let’s do a more typical example problem - something you couldn’t figure out in your

  • head right off. If a man has a mass of 75.0 kg, what is his mass in lbs?

  • Write what you know on the left 75.0 kg

  • and write what you want on the right. = some number of lbs. Leave a little room

  • for the conversion factor. We will multiply by a conversion factor to

  • change the units from kg to lbs. Remember, the conversion factor has to be

  • equal to 1.

  • I would look this conversion factor up, because I probably don’t know this off the top of

  • my head like I do the conversion factor 12 inches for every 1 foot. Okay, so I look it

  • up, and I find that there are 2.2 lb in every 1 kg. Let’s write that as a fraction equal

  • to 1. 2.2lb/1 kg

  • As it turns out, we wrote that fraction in just the right way to solve this problem.

  • Can you see that it would NOT be the right order if I wrote it as 1kg/2.2 lb? That fraction

  • equals 1, but the units wouldn’t cancel in our problem. So let’s take that out and

  • put it back the right way.

  • 75.0 kg x (2.2 lb/1kg) = some number of lbs

  • You can see that the units cancel - kg cancels kg, and we will just be left with lbs, which

  • is what we want to be left with in our final answer. Multiply across the top and bottom.

  • 75.0 x 2.2 = 165.0 lb make sure you write in the units.

  • Don’t forget to check for # of significant figures. We had 3 significant figures in 75.0,

  • so we can write 3 significant figures in our final answer. 165 lb

Unit conversion problems come up a lot in science. Sometimes you hear this style of

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B1 中級

化學。組織、部門換算/維度分析介紹|家庭作業輔導員--------。 (Chemistry: Introduction to Unit Conversion / Dimensional Analysis | Homework Tutor)

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    林宜悉 發佈於 2021 年 01 月 14 日
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