You probably have experience with the first part of this law—objects persisting in a state of rest, meaning objects that are not moving. In fact, you see such objects every moment of every day. But it may best be understood if you make it personal—-like that time you were sitting on the sofa, listening to your favorite music, totally still. You were in a state of "inertia."

Suddenly your brother appeared and decided he wanted to sit in exactly the same spot. He lowered his shoulder and knocked you onto the floor! The force of your brother's muscles in this case made you leave your state of rest. If he had left you alone, you would have persisted in the non-moving state (inertia) until you decided to use the force of your own muscles to stand up or move to a different position. Similarly, an airplane sitting on a runway will not start moving unless some external force is applied to it.

There, that was easy. Now for the harder part—objects in uniform motion in a straight line. From Newton's law, It almost sounds like objects in motion will keep going on forever, doesn't it? But wait—there's that little part about "forces impressed" (forces acting) on the object. Actually, it's a really big part of the story, because these external forces are everywhere.

What Newton was trying to say is that an object moving in a straight line will keep moving in a straight line at the same velocity if no force is applied to it. If you are flying in an airplane in a level, straight line, and have enough fuel to maintain a constant speed, the airplane will continue in a straight line unless some other force acts upon it.

But look out for those external forces, which can come from the atmosphere in the form of turbulent conditions that, if acting on a jetliner, can draw a warning from the pilot. Turbulence applies random forces to an airplane, making it bump up and down and jump left and right, causing the craft to leave its straight path. Similarly, head winds (blowing in the opposite direction to the plane's motion) slow the plane down, and gravitational forces try to pull it toward the Earth. By raising or lowering the flaps on the wings or tail, the pilot can change the direction or speed of the plane. All of these are applied forces that make the plane deviate from its path of uniform motion in a straight line.

Newton's Second Law of Motion
"For a constant mass, force (F) equals mass (m) times acceleration (a)." [Putting this into mathematical form: F=ma.]

Now we are getting into some pretty heavy math. But it's not that bad. Let's walk through this equation step by step.

With his Second Law of Motion, Newton gave us a way to calculate the forces acting on the objects that he talked about in his First Law of Motion.

Suppose a Boeing 747-400 jet weighing 350,000 kilograms is sitting at rest on a runway. To get it moving, some force has to be applied to it, typically the force of a jet engine. Say the pilot revs up the jet engines and the plane accelerates down the runway at 5 meters per second per second.

Whoa! Wait a minute. Is that a typo-per second per second? If not, what does per second per second mean?

Well, first of all, it isn't a typo. Let's look at the term again: 5 meters per second per second. The first part, 5 meters per second, is just a speed, like 60 miles per hour. It has a length term (meters, miles, etc.) and a time term (seconds, hours, etc.) The last "per second" indicates the length of time over which this speed is changing. So 5 meters per second per second means that the jet is increasing its speed by 5 meters per second with every second that goes by. It is accelerating—gaining speed over time. In mathematical terms, acceleration = 5 meters/second/second.

Now we can calculate the force on the jet as it moves down the runway. Force = mass times acceleration. F = ma, where m = 350,000 kilograms and a = 5 meters/second/second. Multiply this out and you get F = 1,750,000 kilogram meters/second/second. To make things easier, and to prevent you from having to say "kilogram meters" and "per second per second" all the time, scientists decided to group all the units together into a new unit called the "Newton." One Newton is 1 kilogram meter/second/ second. So in this example the force on the force on the jet is 1,750,000 Newtons.

It pays to be a famous scientist—maybe if you become one, they'll name a unit after you someday!

Newton's Third Law of Motion
"For every action there is an equal and opposite reaction."

This one is easy. When a NASA rocket is sitting on its launch pad and the countdown reaches zero, rocket fuel ignites and liftoff occurs. Burning gas shoots out the bottom of the rocket in a downward direction, and the rocket moves off the launch pad in an upward direction. The force of the gases moving downward is exactly equal to the force of the rocket moving upward. The "action" (in this case the ignition of the rocket fuel) causes a "reaction" (the liftoff of the rocket) that is equal and opposite.