Newton's laws of motion
The second part is harder to explain. A ball rolling along a flat surface will slow down and eventually come to a halt. This is because of friction, which is a force that slows the ball down. A ball rolling down a slope is also affected by friction, but the effect of the force of gravity is stronger than the friction, so there is a net force acting in the direction in which the ball is moving. In a place without the forces of friction, air resistance and gravity (e.g. in outer space) a moving object would keep on moving in a straight line if there were no force to slow it down or change its direction.