An object will continue to move until a force slows or stops it. The force stopping the object's motion might be an obvious one - the ground! Friction is a force that slows or stops motion. Friction is the resistance to motion created by two objects rubbing against each other (the sled and the snow, for instance).
In general, we can say, two sets are equivalent to each other if the number of elements in both the sets is equal. And it is not necessary that they have same elements, or they are a subset of each other.