Firms' products are perfect substitutes
Firms are "price-takers", no one firm can affect the market price
Market entry and exit are free†
† Remember this feature. It turns out to be the most important!
\(\pi(q)=R(q)-C(q)\)
Graph: find \(q^*\) to max \(\pi \implies q^*\), max distance between \(R(q)\) and \(C(q)\)
\(\pi(q)=R(q)-C(q)\)
Graph: find \(q^*\) to max \(\pi \implies q^*\), max distance between \(R(q)\) and \(C(q)\)
Slopes must be equal: $$MR(q)=MC(q)$$
\(\pi(q)=R(q)-C(q)\)
Graph: find \(q^*\) to max \(\pi \implies q^*\), max distance between \(R(q)\) and \(C(q)\)
Slopes must be equal: $$MR(q)=MC(q)$$
At \(q^*=5\):
Suppose the market price increases
Firm--always setting \(MR=MC\)--will respond by producing more
Suppose the market price decreases
Firm--always setting \(MR=MC\)--will respond by producing less
† Mostly...there is an exception we'll see shortly!
Profit is $$\pi(q)=R(q)-C(q)$$
Profit per unit can be calculated as: $$\begin{align*} \frac{\pi(q)}{q}&=AR(q)-AC(q)\\ &=p-AC(q)\\ \end{align*}$$
Profit is $$\pi(q)=R(q)-C(q)$$
Profit per unit can be calculated as: $$\begin{align*} \frac{\pi(q)}{q}&=AR(q)-AC(q)\\ &=p-AC(q)\\ \end{align*}$$
Multiply by \(q\) to get total profit: $$\pi(q)=q\left[p-AC(q) \right]$$
At market price of \(p^*=\) $10
At \(q^*=5\) (per unit):
At \(q^*=5\) (totals):
## geom_segment: arrow = NULL, arrow.fill = NULL, lineend = butt, linejoin = round, na.rm = FALSE## stat_identity: na.rm = FALSE## position_identity
At market price of \(p^*=\) $10
At \(q^*=5\) (per unit):
At \(q^*=5\) (totals):
At market price of \(p^*=\) $10
At \(q^*=5\) (per unit):
At \(q^*=5\) (totals):
At market price of \(p^*=\) $10
At \(q^*=5\) (per unit):
At \(q^*=5\) (totals):
At market price of \(p^*=\) $2
At \(q^*=1\) (per unit):
At \(q^*=1\) (totals):
At market price of \(p^*=\) $2
At \(q^*=1\) (per unit):
At \(q^*=5\) (totals):
At market price of \(p^*=\) $2
At \(q^*=5\) (per unit):
At \(q^*=5\) (totals):
At market price of \(p^*=\) $2
At \(q^*=5\) (per unit):
At \(q^*=5\) (totals):
What if a firm's profits at \(q^*\) are negative (i.e. it earns losses)?
Should it produce at all?
Suppose firm chooses to produce nothing \((q=0)\):
If it has fixed costs, its profits are:
$$\begin{align*} \pi(q)&=pq-C(q)\\ \end{align*}$$
Suppose firm chooses to produce nothing \((q=0)\):
If it has fixed costs, its profits are:
$$\begin{align*} \pi(q)&=pq-C(q)\\ \pi(q)&=pq-f-VC(q)\\ \end{align*}$$
Suppose firm chooses to produce nothing \((q=0)\):
If it has fixed costs, its profits are:
$$\begin{align*} \pi(q)&=pq-C(q)\\ \pi(q)&=pq-f-VC(q)\\ \pi(0)&=-f\\ \end{align*}$$
$$\begin{align*} \pi \text{ from producing} &< \pi \text{ from not producing}\\ \end{align*}$$
$$\begin{align*} \pi \text{ from producing} &< \pi \text{ from not producing}\\ \pi(q) &< -f \\ \end{align*}$$
$$\begin{align*} \pi \text{ from producing} &< \pi \text{ from not producing}\\ \pi(q) &< -f \\ pq-VC(q)-f &<-f\\ \end{align*}$$
$$\begin{align*} \pi \text{ from producing} &< \pi \text{ from not producing}\\ \pi(q) &< -f \\ pq-VC(q)-f &<-f\\ pq-VC(q) &< 0\\ \end{align*}$$
$$\begin{align*} \pi \text{ from producing} &< \pi \text{ from not producing}\\ \pi(q) &< -f \\ pq-VC(q)-f &<-f\\ pq-VC(q) &< 0\\ pq &< VC(q)\\ \end{align*}$$
$$\begin{align*} \pi \text{ from producing} &< \pi \text{ from not producing}\\ \pi(q) &< -f \\ pq-VC(q)-f &<-f\\ pq-VC(q) &< 0\\ pq &< VC(q)\\ \mathbf{p} &< \mathbf{AVC(q)}\\ \end{align*}$$
Firm's short run (inverse) supply:
Firm's short run (inverse) supply:
1. Choose \(q^*\) such that \(MR(q)=MC(q)\)
1. Choose \(q^*\) such that \(MR(q)=MC(q)\)
2. Profit \(\pi=q[p-AC(q)]\)
1. Choose \(q^*\) such that \(MR(q)=MC(q)\)
2. Profit \(\pi=q[p-AC(q)]\)
3. Shut down if \(p<AVC(q)\)
1. Choose \(q^*\) such that \(MR(q)=MC(q)\)
2. Profit \(\pi=q[p-AC(q)]\)
3. Shut down if \(p<AVC(q)\)
Firm's short run (inverse) supply:
$$\begin{cases} p=MC(q) & \text{if } p \geq AVC\\ q=0 & \text{If } p < AVC\\ \end{cases}$$
Example: Bob's barbershop gives haircuts in a very competitive market, where barbers cannot differentiate their haircuts. The current market price of a haircut is $15. Bob's daily short run costs are given by:
$$\begin{align*} C(q) &= 0.5q^2\\ MC(q) &=q\\ \end{align*}$$
How many haircuts per day would maximize Bob's profits?
How much profit will Bob earn per day?
Find Bob's shut down price.
Example: A firm has short-run costs given by: $$\begin{aligned} C(q)&=q^2+1\\ MC(q)&=2q\\ \end{aligned}$$
Write equations for fixed costs, variable costs, average fixed costs, average variable costs, and for average (total) costs.
Suppose the firm is in a competitive market, and the current market price is $4, how many units of output maximize profits?
How much profit will this firm earn?
At what market price would the firm break even \((\pi=0)\)?
Below what market price would the firm shut down in the short run if it were earning losses?
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