Also known as Newton's first law of motion, the law of inertia states that:

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction (ie, same velocity) unless acted upon by an unbalanced force.

Inertia is the tendency of an object to resist changes in its velocity. 

All objects resist changes in their state of motion. All objects have inertia.

The tendency of an object to resist changes in its state of motion varies with mass.

The more inertia that an object has, the more mass that it has. A more massive object has a greater tendency to resist changes in its state of motion.

Suppose that there are two seemingly identical bricks at rest. Yet one brick consists of mortar and the other brick consists of Styrofoam. Without lifting the bricks, you could give the bricks an identical push in an effort to change their state of motion. The brick that offers the least resistance is the brick with the least inertia - and therefore the brick with the least mass (i.e., the Styrofoam brick).

Speed is a scalar quantity that refers to "how fast an object is moving." Speed can be thought of as the rate at which an object covers distance.

Velocity is a vector quantity that refers to "the rate at which an object changes its position." Velocity can be thought of as the rate at which an object covers displacement. Imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. While this might result in a frenzy of activity, it would result in a zero velocity because this person has had 0 displacement.

Newton's third law is: For every action, there is an equal and opposite reaction.

A force is a push or a pull that acts upon an object as a results of its interaction with another object. Forces result from interactions! Some forces result from contact interactions and other forces are the result of action-at-a-distance interactions.

According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces and are the subject of Newton's third law of motion.

Distance is a scalar quantity that refers to "how much ground an object has covered" during its motion.

Displacement is a vector quantity that refers to "how far out of place an object is"; it is the object's overall change in position.

Suppose you walks 4 meters East, 2 meters South, 4 meters West, and finally 2 meters North. Even though you have walked a total distance of 12 meters, your displacement is 0 meters.

Scalars are quantities that are fully described by a magnitude (or numerical value) alone.
Vectors are quantities that are fully described by both a magnitude (or numerical value) and a direction. For example,  More...

Speed = Distance / Time

Distance = Speed * Time

Time = Distance / Speed

Acceleration is a vector quantity that is defined as the rate at which an object changes its velocity. An object is accelerating if it is changing its velocity.

Newton's second law of motion can be formally stated as follows:

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.

This verbal statement can be expressed in equation form as follows:

The above equation is often rearranged to a more familiar form as shown below. The net force is equated to the product of the mass times the acceleration.

Suppose there are objects out of which there are exactly objects of type and objects of type . Also, suppose there are exactly objects that are of both type AND . Then, how many objects are NOT of type OR ?   More...