⚡ Short Run (Supply Context): Some Factors of Production are Variable
In the short run, a firm faces both fixed and variable factors. Factories cannot expand overnight, but managers can hire extra workers or buy more raw materials. This creates the law of diminishing returns. We explore the relationship between total product (TP), marginal product (MP), and average product (AP). Real‑world bakeries, car washes, and pizza shops show how costs behave. Key ideas: fixed cost (FC), variable cost (VC), marginal cost (MC), and the shape of the supply curve.
🧱 Fixed vs. Variable — The Two Families of Inputs
Imagine a lemonade stand. You already built the stand (fixed); it takes time to build a new one. But you can buy more lemons and sugar (variable) in minutes. Economists call the short run a time window where you cannot change everything. At least one factor — usually capital (machines, buildings) — is stuck. All other inputs (labour, materials, energy) can go up or down.
If you can change it before your next coffee break → variable. If it needs a construction permit → fixed (in the short run).
Let’s meet Maria. She runs a small pottery workshop with two kilns (fixed). She can hire 1, 2, or 10 assistants (variable). The table below shows what happens when she adds more labour to the same two kilns.
| Workers (L) | Total pots (TP) | Marginal product (MP) | Average product (AP) |
|---|---|---|---|
| 0 | 0 | — | — |
| 1 | 10 | 10 | 10.0 |
| 2 | 25 | 15 | 12.5 |
| 3 | 34 | 9 | 11.3 |
| 4 | 40 | 6 | 10.0 |
| 5 | 42 | 2 | 8.4 |
Maria sees a pattern: the marginal product first rises (specialisation), then falls (crowding). That is the famous law of diminishing marginal returns — a short‑run truth. More workers with the same kilns eventually add less and less extra output.
💰 From Production to Costs — The Wallet Side
Every input has a price. Fixed factors give fixed cost (FC) — rent, insurance, machine payments. Variable factors give variable cost (VC) — wages, flour, electricity. Add them to get total cost (TC): $TC = FC + VC$.
For a school bake sale, the oven rental is $20 (fixed). Each batch uses $5 of ingredients (variable). If you bake 10 batches, VC = $50, TC = $70. The cost per batch? That is average total cost (ATC): $ATC = \frac{TC}{Q}$.
| Q (cakes) | FC ($) | VC ($) | TC ($) | MC ($) | ATC ($) |
|---|---|---|---|---|---|
| 0 | 20 | 0 | 20 | — | — |
| 5 | 20 | 25 | 45 | 5.0 | 9.0 |
| 10 | 20 | 45 | 65 | 4.0 | 6.5 |
| 15 | 20 | 70 | 90 | 5.0 | 6.0 |
| 20 | 20 | 100 | 120 | 6.0 | 6.0 |
Notice that marginal cost (MC) first falls, then rises — the mirror image of marginal product. When MP is high, MC is low. When MP drops, MC climbs. $MC = \frac{\Delta TC}{\Delta Q}$.
🍕 Friday Night Pizza Rush — Short Run in Action
Tony’s Pizzeria has four ovens (fixed). On a normal Tuesday he uses two cooks (variable). Friday night demand explodes. He cannot install new ovens before Saturday, but he can hire three more cooks, ask the cashier to help with toppings, and buy extra cheese. This is the short run. Output rises, but the kitchen becomes crowded. The 5th cook keeps bumping into others — marginal product per cook drops. Tony’s marginal cost per pizza goes up. He still supplies more because the price is high, but his profits per pizza shrink.
This real example explains why supply curves slope upward in the short run: to produce more, firms need higher prices to cover the rising marginal cost caused by fixed factors.
❓ Three Questions Students Always Ask
A: No! It is different for every industry. For a food truck, the short run might be one day (you need weeks to buy a new truck). For a nuclear power plant, the short run could be five years (building a reactor takes a decade). The rule: at least one factor is fixed.
A: Because of the law of diminishing returns. When you add more variable inputs to a fixed input, each new unit adds less output. If each worker costs the same wage, but adds fewer pizzas, the cost per extra pizza increases. Simple: less output for the same money = higher cost.
A: Not really. By definition, the short run includes at least one fixed factor. Even a freelance tutor has a fixed cost: her laptop or her internet subscription. If every cost were variable, we would be in the long run.
The short run is not a flaw; it is reality. Firms cannot teleport new factories. Understanding that some inputs are sticky explains why supply reacts slowly, why marginal costs curve upwards, and why managers obsess over productivity. From a lemonade stand to a smartphone assembly line, the short run reminds us that time is the ultimate constraint. When you see a “help wanted” sign or a “limited time offer” — that is the short run calling.
📌 Footnote — Abbreviations & Terms
VC: Variable Cost — cost that varies directly with output (e.g., raw materials).
TC: Total Cost — sum of FC and VC.
MC: Marginal Cost — extra cost from producing one more unit.
TP, MP, AP: Total, Marginal, and Average Product — measures of output per input.
Short run (economic definition): a period where at least one factor of production is fixed; firms can change variable inputs but not all inputs.