The momentum of a roller skater is not conserved because what acts on the skates?

The momentum of a roller skater is not conserved because external forces act on the skates, primarily friction and air resistance. These forces can change the skater's velocity, thus affecting their momentum. Additionally, when the skater pushes off the ground or interacts with other objects, those actions can also alter their momentum. Therefore, while momentum can be conserved in an isolated system, the skater is subject to external influences that disrupt this conservation.