What is Newton's Law of Motion - It's Introduction, 1st, 2nd & 3rd Law with Examples

Introduction:

If we push a chair across the floor, what causes the chair to move or to stop moving? Newton’s first two laws of motion address these questions and, in the process, provide part of a definition of force. Where do forces come from? If you push on a chair to move it across the floor, does the chair also push back on you? If so, how does that push affect your own motion? Newton’s third law provides some answers.

The first law tells us what happens in the absence of a force, and the second describes the effects of applying a force to an object. Third law tells us that forces are caused by such interactions of two objects, each exerting a force on the other.

Together, these three laws describe almost everything about how and why objects in our world move the way they do.

Newton’s first law of motion:

Newton’s first law of motion can be stated as follows: An object remains at rest, or in uniform motion in a straight line, unless it is compelled to change by an externally imposed force. It's also known as The Law of Inertia.

The acceleration of an object is directly proportional to the magnitude of the imposed force and inversely proportional to the mass of the object. The acceleration is in the same direction as that of the imposed force.

In other words, unless there is a force acting on the object, its velocity will not change. If it is initially at rest, it will remain at rest; if it is moving, it will continue to do so with constant velocity.

Constant velocity implies that neither the direction nor the magnitude of the velocity changes. When the object is at rest, its velocity is zero, and that value remains constant in the absence of an external force. If there is no force acting on the object, the acceleration of the object is zero. The velocity does not change.

Objects are lazy. They don't like change. A still object will just sit there forever. A moving object will keep sliding in a straight line forever. The only reason they don't is because a force gets in the way usually friction or gravity.

You're in a car that stops suddenly. Your body moves forward. Why? The car stopped, but you were in motion and wanted to stay in motion. Your body's inertia is what causes you to keep moving until the seatbelt another force stops you.

Newton’s second law of motion:

Newton’s second law of motion is a more complete statement about the effect of an imposed force on the motion of an object. Stated in terms of acceleration, it says

The acceleration of an object is directly proportional to the magnitude of the imposed force and inversely proportional to the mass of the object. The acceleration is in the same direction as that of the imposed force.

This statement is most easily grasped in symbolic form. By choosing appropriate units for force, we can state the proportionality of Newton’s second law as the equation,

a = Fnet/m

where a is the acceleration, Fnet is the total, or net, force acting on the object, and m is the mass of the object. Because the acceleration is directly proportional to the imposed force, if we double the force acting on the object, we double the acceleration of the object. The same force acting on an object with a larger mass, however, will produce a smaller acceleration.

This is the law of "cause and effect" for movement. The Force is how hard you push or pull something. The Mass is how much weight is in the object. The Acceleration is how much it speeds up, slows down, or changes direction.
For the same amount of push, a lighter thing moves a lot more than a heavier thing.

e.g. Pushing an empty shopping cart vs. a full one. You push both with the same force. The empty cart with less mass moves away with quickly means high acceleration. The full cart with more mass barely moves means low acceleration.

Learn more about in Force.

Newton’s third law of motion:

If you push with your hand against a large chair or any other large object, such as the wall of your room, you will feel the object push back against your hand. A force is acting on your hand that you can sense as it compresses your hand. Your hand is interacting with the chair or wall, and that object pushes back against your hand as you push against the object.

Newton’s third law contains the idea that forces are caused by such interactions of two objects, each exerting a force on the other. It can be stated thus:

Forces always come in pairs. If Object A pushes on Object B, then Object B immediately and simultaneously pushes back on Object A with the exact same amount of force, but in the opposite direction.

The third law is also defined as, for every action there is an equal but opposite reaction. The two forces always act on two different objects, never on the same object. This is why they don't cancel each other out.

e.g. If you exert a force F1 on the box with your hand, the box pushes back on your hand with a force F2 that is equal in size but opposite in direction. Using this notation, Newton’s third law can be stated in symbolic form as, F2 = −F1

The minus sign indicates that the two forces have opposite directions. The force F2 acts on your hand and partly determines your own motion, but it has nothing to do with the motion of the box. Of this pair of forces, the only one that affects the motion of the box is the one acting on the box, F1.