Second Term Lesson Note for Week Eight
Class : JSS 2
Subject : Mathematics
Topic : Scale Drawing
Duration : 80 Minutes
Period : Double Period
Reference Book :
- Essential Mathematics for Junior Secondary School, JSS 2.
- Lagos State Unified schemes of work for Junior Secondary Schools, JSS 1 – 3.
- Online Resources
Instructional Material : Chart showing the rules and procedure used in scale Drawing.
Learning Objectives : By the end of the lesson learners will be able to :
i. Explain Scale drawing
ii. Enlargement of an image using scale
iii. Reduction of an image using scale drawing.
Content :
SCALE DRAWING
Since it is not always possible to draw on paper the actual size of real-life objects such as the real size of a traingle, an airplane, we need scale drawings to represent the size like the one you see below of a triangle.
In real-life, the length of this triangle may measure 30 inches. However, the length of a copy or print paper that you could use to draw this van is a little bit less than 10 inches
Since 30/10 = 3, you will need about 3 sheets of copy paper to draw the length of the actual size of the triangle
In order to use just one sheet, you could then use 1 inch on your drawing to represent 3 inches on the real-life object
You can write this situation as 1:3 or 1/3 or 1 to 3
Notice that the first number always refers to the length of the drawing on paper and the second number refers to the length of real-life object
In the above rectangle, the actual size is length 3 and breadth 2.
To reduce the size to fit in, we use a scale factor of 1/2, divide the length and breadth by 2 to give the new length as 1.5 and breadth 1.
Also, to enlarge the size of the rectangle, we use a scale factor of 2, multiply the length and the breadth by 2 to give new length as 6 and breadth as 4 respectively.
Example 1:
Suppose a problem tells you that the length of a vehicle is drawn to scale. The scale of the drawing is 1:20
If the length of the drawing of the vehicle on paper is 12 inches, how long is the vehicle in real life?
Set up a proportion that will look like this :
Do a cross product by multiplying the numerator of one fraction by the denominator of the other fraction
We get :
Length of drawing × 20 = Real length × 1
Since length of drawing = 12, we get:
12 × 20 = Real length × 1
240 inches = Real length
Example 2:
The scale drawing of this tree is 1:500
If the height of the tree on paper is 20 inches, what is the height of the tree in real life:
Set up a proportion like this:
Do a cross product by multiplying the numerator of one fraction by the denominator of the other fraction
We get :
Height of drawing × 500 = Real height × 1
Since height of drawing = 20, we get:
20 × 500 = Real length × 1
10000 inches = Real height
Evaluation :
1. A house size Length 120m by 60m is to be drawn on an A3 sheet of paper using 1:30. What is the size of the house on the paper?
2. The length of a pole on a paper is 50cm, if the scale factor used is 1 : 50. what is the original length of the pole?
Conclusion : At the end of the lesson learners were able to answer the questions correctly.