Final answer:
When roller skating on level pavement, you can maintain your speed for a long time. However, when skating up a gradual hill, you begin to slow down due to the force of gravity and friction. Gravity pulls you downhill, and friction between your skates and the pavement opposes your motion, causing you to lose kinetic energy.
Step-by-step explanation:
When you are roller skating on level pavement, you can maintain your speed for a long time because there is minimal resistance to your motion. However, when you start skating up a gradual hill, you begin to slow down because of the force of gravity acting against you. This force of gravity is pulling you downhill, and as you skate uphill, you have to exert more effort to overcome the force of gravity and maintain the same speed.
Friction between your skates and the pavement also plays a role in slowing you down. Friction is the force that opposes the motion of objects sliding against one another. As you skate uphill, the friction between your skates and the pavement increases, which causes you to lose some of your kinetic energy and slow down.
To summarize, when you skate on level pavement, the minimal resistance to your motion allows you to maintain your speed. However, when you skate uphill, gravity and friction work together to slow you down.