Why an empty cart on a roller coaster has a lower mass than a full cart?

An empty cart on a roller coaster has a lower mass than a full cart because it lacks the additional weight of passengers. Mass is a measure of the amount of matter in an object, and without the bodies of riders, the empty cart contains only its own structural components. Therefore, the total mass of the empty cart is simply the sum of its material, while a full cart includes both the cart's mass and the mass of the passengers.