Newton's Laws of Motion

The focus here is Newton's first law of motion - sometimes referred to as the law of inertia.
Newton's first law of motion is often stated as :
An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction-unless acted upon by an unbalanced force.
Everyday Applications of Newton's First Law
There are many applications of Newton's first law of motion. Several applications are listed below. Perhaps you could think about the law of inertia and provide explanations for each application.
Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.
The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.
A brick is painlessly broken over the hand of a physics teacher by slamming it with a hammer.
To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and thrusted downward at high speeds and then abruptly halted.
Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.
While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object which abruptly halts the motion of the skateboard.
Inertia and Mass
Newton's first law of motion states that "An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force." Objects tend to "keep on doing what they're doing." In fact, it is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion is described as inertia.
Galileo and the Concept of Inertia
Galileo, it premier scientist in the seventeenth century, developed the concept of inertia. Galileo reasoned that mQving objects eventually stop because of a force called friction.
Mass as a Measure of the Amount of Inertia
All objects resist changes in their state of motion. All objects have this tendency - they have inertia. But do some objects have more-ofa tendency to resist changes than others? Absolutely yes! The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity which is solely dependent upon the inertia of an object. The more inertia which an object has, the more mass it has. A more massive object has a greatertendency to resist changes in its state of motion.
Force and Its Representation
The Meaning of Force
A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction.
For simplicity sake, all forces (interactions) between objects can be placed into two broad categories:
contact forces, and
forces resulting from action-at-a-distance
Contact forces are those types of forces which result when the two interacting objects are perceived to be physically contacting each other. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance forces, and applied forces.
Action-at-a-distance forces are those types of forces which result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation. Examples of action-at-adistance forces include gravitational forces. For example, the sun and planets exert a gravitational pull on each other despite their large spatial separation. Even when. your feet leave the earth and you are no longer in physical contact with the earth, there is a gravitational pull between you and the Earth. Electric forces are action-at-a-distance forces. For example, the protons in the nucleus of an atom and the electrons outside the nucleus experience an electrical pull towards each other despite their small spatial separation. And magnetic forces are action-at-a-distance forces. For example, two magnets can exert a magnetic pull on each other even when separated by a distance of a few centimeters.
Examples of contact and action-at-distance forces are listed in the table below.
Contact Forces Action-at-a-Distance Forces
Frictional force Gravitational Force
Tension force Electrical Force
Normal Force Magnetic Force
Air Resistance Force
Applied Force
Spring Force
Force is a quantity which is measured using the standard metric unit known as the Newton. A Newton is abbreviated by a "N." To say "10.0 N" means 10.0 Newtons of force. One Newton is the amount of force required to give a 1-kg mass an acceleration of 1 m/s/s. Thus, the following unit equivalency can be stated:
1 Newton = 1 kg* m/s2
A force is a vector quantity.
Newton's Second Law of Motion
The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
The net force is equated to the product of the mass times the acceleration.
Fnet = m * a
Newton's Third Law
Formally stated, Newton's third law is:
For every action, there is an equal and opposite reaction.
The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.
Projectile Motion
What is a Projectile?
The most common example of an object which is moving in two dimensions is a projectile.
A projectile is an object upon which the only force acting is gravity. There are a variety of examples of projectiles. An object dropped from rest is a projectile (provided that the influence of air resistance is negligible). An object which is thrown vertically upward is also a projectile (provided that the influence of air resistance is negligible). And an object is which thrown upward at an angle to the horizontal is also a projectile (provided that the influence of air resistance is negligible). A projectile is any object which once projected or dropped continues in motion by its own inertia and is influenced only by the downward force of gravity.
By definition, a projectile has only one force actiing upon it - the force of gravity. If there was any other force acting upon an object, then that object would not be a projectile.