Suppose you find yourself teetering on the edge of a cliff or the roof of a tall building with a heavy physics book in your hand. as you are about to fall, you suddenly remember how physics can save you! what law did you remember?

Law of Conservation of Momentum

Explanation:

According of Law of Conservation of Momentum:

"If the result of the forces acting on a body or system is null, its linear momentum remains constant in time ".

In fact, this law is actually a direct consequence of Newton's third law of motion:

"When two bodies interact between them, appear equal forces and opposite directions in each of them."  

To understand it better:  

Each time a body or object exerts a force on a second body or object, it (the second body) will exert a force of equal magnitude and direction but in the opposite direction on the first.

So, if you If you throw the heavy physics book forward, you will have a backward momentum (with the same magnitude and opposite direction), hence a backward force that will make you fall to the side of the building and you will not fall over the edge.

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me