The answer would be:
Newton's first law of motion, also known as the Law of Inertia
Here's more about it:
Isaac Newton came up with three laws of motion:
Law of Inertia
Law of Acceleration
Law of Reaction
The law the question is pertaining to is the law of inertia which states:
"An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force."
The main thing you should remember is the phrase "unless acted upon by an unbalanced force."
Let's say you have a book on the counter. It is at rest, if it is not moving. Now how will you move that? You are going to move it by applying force on the book in the direction you want to move it. Unless you apply force enough to move it, it will stay in its position.
Let's say you have a truck. Unless you can apply enough force to move it, it will stay at rest.
Now if you have a moving object, that object will stay in motion and in the same direction unless an unbalanced force will be applied to it. So why does a rolling ball eventually stop? Well the friction and gravity both apply forces on the ball, so it comes out as an unbalanced force.
If the ball were moving in space where both gravity and friction have nothing on it, then it will move continuously until something will stop it.
(This is why astronauts are extra careful when they traverse space and make sure that they're attached carefully on the ship because they can drift off without stopping if they're not secured.)
