When the average kinetic energy of the molecules goes up (a rise in temperature), the average speed of the molecules increases. And lower average kinetic energy of the molecules means they have lower speed. However, a change in average kinetic energy is not directly proportional to a change in average speed. 1.
An appreciation means an increase in the value of a currency. It means a currency is worth more in terms of foreign currency.
Foreign governments were able to take their U.S. currency and exchange it for gold with the U.S. Federal Reserve. If we think about this relationship between money and gold, we can gain some insight into how money gains its value: like the beaver pelts and dried corn, gold is valuable purely because people want it.