Final answer:
A fixed quantity supplied in the short run signifies a perfectly inelastic supply curve, indicating zero change in quantity supplied despite price fluctuations. Perfect elasticity, marked by a horizontal supply curve, is its opposite, where the quantity supplied is highly responsive to price changes. Both are extreme cases of elasticity with most goods' supply elasticity falling between these extremes.
Step-by-step explanation:
If the quantity supplied of some good is fixed in the short run, resulting in a vertical supply curve, the elasticity of supply for that good is perfectly inelastic. This means that there will be zero percentage change in the quantity supplied, regardless of the price level. A vertical supply curve indicates that no matter how much the price changes, the quantity of the good that producers are willing to supply remains constant. This could be due to production constraints, legal limitations, or the physical unavailability of additional resources necessary to increase production.
It is important to note that a perfectly inelastic supply is one of the extreme cases of elasticity; the other extreme is perfectly elastic supply, which is characterized by a horizontal supply curve. Perfect elasticity signifies that the quantity supplied can change by an infinite amount in response to any change in price. However, in reality, situations of perfect elasticity and perfect inelasticity are rare, with most goods falling somewhere in between these two extremes.