What causes an object to speed up or slow down? Well, the object must be acted on by a force. A force is simply just a push or a pull. Try pushing your notebook across your desk. The notebook moves because you used the force from your hand to speed it up. When you stopped pushing the notebook, it stopped also. There is not a force to make it move anymore.
Forces are measured in Newtons. One Newton (1N) is the force that produces an acceleration of 1 meter per second per second when exerted on a mass of 1 kilogram. The Newton is named for Sir Isaac Newton.
Whenever two or more forces act upon an object in a way as to exactly counteract each other (equal in size and opposite in direction) they are balanced and they stay put. When they are unbalanced, they move.