Consider the supply of shipping services. The law of supply suggests that as the price of shipping increases, the quantity of shipping services will increase. At a relatively low freight rate of $2 per ton, only the most efficient ships operate, and they economize on fuel by traveling at a relatively low speed. As a result, the annual quantity of shipping services is only 70!units per year. At an intermediate freight price of $3 per ton, less efficient ships join the fleet. In addition, all the ships travel at a greater speed, using more fuel in the process. The combination of more ships and faster travel increases the quantity of shipping services provided to 85 units per year. At a high freight rate of $7 per ton, the least efficient ships join the fleet and all the ships run at full speed. As a result, the quantity of shipping services increases to 96 units per year.

Question.

Why is the short-run supply curve positively sloped?