Hi guys, I thought of something when I was rolling something. In a nut shell, why is it easier to roll an object once it started rolling, and what would be the force required to keep the object rolling? Let's use a car as an example, when you push the car, it will only start rolling once you apply more force than the static friction force. But once the car start rolling, it becomes increasingly easier for you to keep the car rolling (hence need less apply force). I think the reason for this is because the car will have more and more momentum as it roll faster and faster, thus the faster the car is rolling, the less force is needed for you to keep it rolling. But then that would lead me to think that there must be an equation which relate the force, momentum and static friction. Does anyone know what that equation is, and is my reason for less force required to roll an object already in motion correct? Thank you!