Here’s a great everyday use of the physics described in this chapter. If you are trying to get ketchup out of the bottle, the best way to do it is to turn the bottle upside down and give the bottle a sharp upward smack, forcing the bottle rapidly upward. Think about what subsequently happens to the ketchup, which is initially at rest, and use Newton’s first law to explain why this technique is so successful.

Because of Newton's first law of motion, or the law of intertia, when you hit the ketchup bottle, your hand being the unbalanced force, it interrupts it's inertia and causes the ketchup to fall into the cap.

Explanation:

Newton's first law is the law of motion, or the law of inertia. An object at rest stays at rest and an object in motion stays in motion unless acted upon by an unbalanced force. If you did not smack the bottle, the bottle would just keep staying in motion. But when you smack the bottle, the motion stops (your hand being the unbalanced force) and the force of it causes the ketchup to fall into the cap.