Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

The automatically straight-line graph

Examples of using a straight line graph to find a formula. 
 
1 To show that πR2 gives the area of a circle. 
 
For any circle π is the number 3.14 in the equation: 
circumference = 2πx radius or π x diameter. 
 
So π is circumference/diameter 
 
Starting from that (as a definition of π) we can show that the area of a circle is πR2. 
 
Draw a large circle with centre 0 and radius R. Plot a graph of 2πr upwards against r along. 
 
Then the graph must be a straight line and its slope will be 2π.

Plot a graph of 2πr upwards against r along

2πr

The end-point A, of the graph belongs to a big circle of radius R. Each other point of the graph - line 0A belongs to a smaller circle, of radius r
 
Sketch III shows two small circles close together with radii (r) and (r + tiny bit of radius). 
 
What is the area of the shaded ring between them? The ring has width (tiny bit of radius) andlength 2πr (its circumference). Its area is 2πr x (tiny bit of radius)

Its area is 2πr x (tiny bit of radius

Graph to show 2πr x (tiny bit of radius).

On the Graph IV the shaded pillar shows just that same area, 2πr x (tiny bit of radius).

Now ask about all such rings from the centre 0 out to radius R. Their total area is the same as the area of all the pillars in Graph V. That is the triangle of height 2πR and baseR

That is the triangle of height 2πR and baseRAREA = 1/2 2π R x R = πR2. 

Therefore area of circle is πR2. 
 
2 To show that s = 1/2 at2 for constant acceleration from rest. 

s = 1/2 at2

Plot a graph of at upwards against t along. Then with a constant the graph must be a straight line; and its slope will be a (Graph VI). 

Plot a graph of at upwards against t along

Choose a tiny bit of time on the t-axis and draw a pillar up to the line (Graph VII). The area of the pillar is: height x width

 (at) x (tiny bit of time) 
and that is (v) x (tiny bit of time), since acceleration x time is speed. 

hen total distance travelled, s, is given by the total area of all such pillars

And that is (tiny bit of time travelled). 

Then total distance travelled, s, is given by the total area of all such pillars (Graph VIII). 
 
s = area of triangle OAB 
= 1/2 at x t = 1/2 at2.