How does the law of supply work and why does it slope upward?

The law of supply states that, other things being equal, as the price of a good rises, the quantity that sellers are willing to supply also rises, and when the price falls, supply falls. That is why the supply curve slopes upward from left to right. The reason is profit. When the price is high, each unit sold earns more, so producers are motivated to make and sell more to earn greater profit. Higher prices may also make it worthwhile for new sellers to enter the market. When prices are low, profits shrink, so sellers offer less. Note the phrase 'other things being equal': this assumes costs of production, technology and other factors stay constant. So demand and supply move oppositely with price, and the price where they meet is the equilibrium price.