Main content
Early math review
Course: Early math review > Unit 7
Lesson 3: Comparing and estimating lengthComparing lengths
Sal uses a ruler to compare the lengths of different objects.
Want to join the conversation?
Here is something interesting:
I have a holiday from the 10th until the 20th of January. How many days off have I got?
20-10 = 10
But if you count the number of days with your hands, you discover that you have 11 days off!
Can anyone explain this sorcery?(8 votes)
This is because you really start counting from zero.
When you normally count it goes like 1 2 3 4 5 6 7 8 9 10, which is 10 numbers
If you count from zero it is: 0 1 2 3 4 5 6 7 8 9 10, which is 11 numbers
In this case you count from 10th like this: 10 11 12 13 14 15 16 17 18 19 20 and it resembles counting from zero (if you ignore number in ten's position).
When you have holiday from 1st to 10th, it is 10 days.
When you have holiday from 10th to 20th, it is 11 days because you have to add the 10th day.
In fact, you have to add 1 every time when you count the number of days or anything where you count in the starting number of range.
For example:
10 - 1 = 9, but 1 to 10 is 10 numbers
25 - 4 = 21, but 4 to 25 is 22 numbers(21 votes)
how many inches does a yard stick have ?(4 votes)
A yardstick consists of 3 feet, and each foot has 12 inches, so 3 * 12 = 36 inches,
36 inches are in a yardstick(14 votes)
I've got a question. In the first example, Sal compares the width of the 1st rectangle which is 3m with the length of the 2nd rectangle which is 9m. And he said "rectangle NO.1 has a width of 3m and rectangle No.2 has a width of 9m". I want to ask if we distinguish length from width in Maths or in talking about shapes(8 votes)- Typically, we reference the dimension that is most parallel with the directions left and right when looking at four sided objects as width, and the up/down sides are referred to as height. But technically, you could refer to them the other way around if you wanted to. You must be consistent, however, if you are comparing multiple objects.(4 votes)
I will never understand this. Can someone explain?(2 votes)
You compare the two lengths of the objects by first ensuring that both objects are measured in the same measuring unit, such as centimeters. You then can compare the lengths. The object with the bigger number is longer than the object with the smaller number.
Here's an example:
Rectangle A is 5 centimeters long.
Rectangle B is 8 centimeters long.
Compare the length of the two rectangles.
Rectangle A _ Rectangle B
5 centimeters _ 8 centimeters
8 is a larger number than 5, so..
5 centimeters < 8 centimeters.
Therefore,
Rectangle A < Rectangle B
Because 8 is greater than 5, rectangle B is longer than rectangle A.(6 votes)
what would happen if you cant use a ruler to measure? I have watch another video and you had to measure with a rectangle or little squares is that the only way or there can be more?(3 votes)
You measure one thing by comparing it to another thing that you know the length of. So there are loads of things you can use to measure stuff. The only things you wouldn't want to use are things that change length like springs, and things that have wiggly edges - unless the thing you're trying to measure has a wiggly edge too! Good question.(4 votes)
Why is there a circle on the ruler?
Can you measure without a ruler?(1 vote)
Yes, you can. For example, if you wanted to measure how tall a sunflower is, I would use a yardstick.
( That circle is for moving the ruler. )(1 vote)
- What if you're told to measure the sides, not the bottom? Is it still the same concept?(1 vote)
- Yes, just put the ruler along the part you want to measure.(0 votes)
Video transcript
- Rectangle 1 is blank meters
blank than Rectangle 2. Let's look at this. Rectangle 1 has a width of 3 meters. Rectangle 2 has a width of 9 meters. Rectangle 1 is, if you compare 3 to 9, it's going to be 6 meters shorter. 6 meters shorter than Rectangle 2. You see that right over here, because 3 meters is 6 less than 9 meters. It's 6 meters, and you know it's not going to be longer. Rectangle 1 is definitely ... it has less width. It's definitely shorter than Rectangle 2. It's 6 meters shorter than Rectangle 2. Let's do a few more of these. Line 1 is blank centimeters long. We can measure it. We could measure it right over here. Line 1 is 9 centimeters long. 9 centimeters long. Line 2 is blank centimeters long. Let me actually zoom this in a little bit so that we can see it a little bit better. Line 2 is blank centimeters long. Line 2 is 5 centimeters long. Let me zoom in a little bit more even. Line 2 is 5 centimeters long. Line 1 is blank centimeters
blank than Line 2. Well, Line 1 is 9 verses 5, so it's 4 centimeters longer than Line 2. Line 1 is 4 centimeters
longer than Line 2. You can see it right over here. To go from Line 2 to Line 1, you'd have to add 1, 2, 3, 4 centimeters. Let's do a few more of these. Triangle 1 is blank centimeters long. We can just use a ruler to measure that it's 3 centimeters long. Triangle 2 is blank centimeters long. Let me scroll down to look at Triangle 2. Triangle 2 is 5 centimeters long. Triangle 2 is 5 centimeters long. Triangle 1 is blank centimeters
blank than Triangle 2. Well, Triangle 1 is 2 centimeters
shorter than Triangle 2. 3 is 2 less than 5. It's going to be 2 centimeters shorter. Remember, Triangle 1, we see it's shorter it's smaller than Triangle 2, so it makes since that it's shorter. How much shorter? 2 centimeters because 3 centimeters compared to 5 centimeters. Let's do one more of these. Rectangle 1 is blank centimeters long. We can just look right over here. It's 9 centimeters long. We have one end of the ruler
at one end of the rectangle, and then we just measure
off 9 centimeters. Rectangle 2 is, well, it looks like it's 6 centimeters long. 6 centimeters long. Rectangle 1 is longer, and it's 3 centimeters
longer than Rectangle 2. 9 centimeters is 3 centimeters
more than 6 centimeters. It's 3 centimeters longer. Rectangle 1 is the longer Rectangle. Then, we're done.