Supply Equation: Y=mx+c or S=d*Qty+c, Why P Instead of Qty?

[shivajikobardan]

Homework Statement

straight line equation

Relevant Equations

y=mx+c

The equation for supply should be this-:

y=mx+c

S=m*Qty+c
=c+d*Qty

Why is P used instead of Qty?
I understand that supply increases as price increases relation is used here, but I just want to think graphically. This looks wrong according to that.

 

[Delta2]

The weird with the diagram is (I think) that it has the axes swapped. The quantity should be on the y-axis and the price on the x-axis.

 

[shivajikobardan]

The weird with the diagram is (I think) that it has the axes swapped. The quantity should be on the y-axis and the price on the x-axis.

this is same in almost every textbooks about this book.


it looks like 1 person wrote sth and everyone copied it. but since everywhere it's written the same, i suspect there's some logic behind it.

 

[Delta2]

The diagram isn't wrong , it just isn't done the usual way we do in math. It needs a 90 degree counterclockwise rotation.

 

[shivajikobardan]

The diagram isn't wrong , it just isn't done the usual way we do in math. It needs a 90 degree counterclockwise rotation.

i mean how can that be possible? so u r trying to say the x-axis is the y-axis and vice versa, right? can we just assume that?

 

[Delta2]

i mean how can that be possible? so u r trying to say the x-axis is the y-axis and vice versa, right? can we just assume that?

Yes that's what I am saying, the axis are swapped. It looks like instead of graphing for example $D=a-bP$ they have graphed the equivalent $P=\frac{1}{b}(a-D)$.

 

[Hall]

The weird with the diagram is (I think) that it has the axes swapped. The quantity should be on the y-axis and the price on the x-axis.

I just wanted to say that in Economics it is always the Quantity that comes on the x-axis and Price on y-axis.

We tend to think that "at price $x$ we would buy $y$ quantity of goods", but this system of graphing was developed when the classical model was prominent, that is "supply creates its own demand" and that "supply depends only on natural resources available (including labourers)", so suppliers used to decide the price, "Okay, we will sell our products at this price, eh?". It's more of Macroeconomic model, and then was taken in Microeconomics without any change.

 

[pbuk]

I just wanted to say that in Economics it is always the Quantity that comes on the x-axis and Price on y-axis.

Yes, that is exactly right. So whilst in maths we are used to the relationship y = mx + c, where m is the slope and c the y-intercept, in this case it helps to consider instead x = m'y + c' where m' is the reciprocal of the slope and c' is the x-intercept.