Galileo used inclined planes to determine the acceleration
of falling objects and developed the concept that, without friction, a
moving object on a flat plane would continue in a straight line forever
at the same velocity -- Galileo called this “tendency” of matter **inertia**.

Inertia is a property of matter, the tendency of an object to remain in its current state of motion; the resistance of an object to a change in its velocity.

The amount of inertia an object has depends on how much
matter it has, or its **mass**. Mass is simply a measure of how resistant
on object is to changing its state of motion. Mass is also defined as the
amount of matter in an object.

Galileo recognized that the reason a rolling cannonball
eventually came to rest was due to a resistive **force** - **friction**.
Since friction was always present in daily life it had been overlooked
in previous explanations of motion.

Isaac Newton developed three laws of motion based on the
overcoming of an object’s inertia. In his **first law of motion** he
states:

Every body perseveres in its state of rest, or of uniform motion in a right line,

unless it is compelled to change that state by forces impressed thereon.

A force is simply a push or a pull. The force of **gravity**
is the pull due to an object’s mass and its **distance** from another.
Near the earth’s surface gravity overcomes the inertia (mass) of an object
at a rate of 9.8 m/s^{2} (which we call ** g**) towards
the earth.

If there is no acceleration then the net force must be zero (balanced forces), this can happen if an object is (a) not moving, static equilibrium, or (b) moving at a constant velocity, dynamic equilibrium.

As you sit in your seat you have a net force of zero acting on you. If gravity is pulling down on you there must be a balancing force pushing back up on you, thereby resulting in no acceleration.

When you push a chair across the room at a constant velocity the chair slides over the floor. There is an opposing force of friction that must be balanced by your pushing force.

**Friction** is defined as the opposing force that
results when surfaces slide or tend to slide past one another. This includes
solids, liquids, and gases. Static friction is greater than sliding friction.
For solids the speed and surface area have no effect on the friction, but
in fluids friction is effected by **speed** and **surface area**.
This is due to the total amount of fluid pushed aside by the object. Sort
of like trying to walk through the hallways between classes, the faster
you go the more people you encounter in a given time (more opposition)
and the smaller you are the easier it is to avoid people (less opposition).

**Newton’s second law of motion** relates the acceleration
of an object to the “impressed force:”

The alteration of motion is ever proportional to the motive force impressed: and is

made in the direction of the right line in which that force is impressed[,and is

inversely proportional to the mass of the object].

With this in mind a force can also be defined as anything that can accelerate an object.

The force mentioned in the second law refers to a **net
force**. A net force is the overall result of all forces that may be
acting on an object.

This provides an explanation for why it is harder to push a heavy object than a light object.

**Newton’s third law of motion** is probably the best
recognized and the least understood:

To every action there is always opposed an equal reaction: or the mutual actions of

two bodies upon each other are always equal, and directed to contrary parts.

The forces involved here are often referred to as action and reaction where the reaction does not exist without the action.

The misunderstanding of this law lies in the fact that the action force and the reaction force are acting on different objects, and therefore are not balancing forces that result in no acceleration.

This is why a rocket can accelerate in empty space.

**Freefall** is the situation when the only force (and net force)
acting on an object is that of gravity, this is when the frictional forces
in the atmosphere are negligible.

One of Galileo’s observations was that during freefall the acceleration
on any object is a constant value ** g**, regardless of the object’s
mass. This is easily proved with the help of Newton’s second law of motion
and the fact that the weight of an object is the force required to accelerate
an object at

Basically the 20 kg brick has twice the inertia of the 10 kg brick and therefore requires twice the force, and always resulting in the same acceleration because of the equal ratios of force to mass. When observing differing mass objects in the classroom it is almost impossible to detect the effect of air resistance, but it is still there. The presence of the air resistance causes a lower net force pulling down on an object, but this net force is not constant.

This real life condition is known as **non-freefall.**

Remember that the size of the fluid friction force depends on surface
area and velocity. The surface area can be changed, but does not always.
However, the velocity is always changing when there is a net force, and
as the velocity increases the force of air resistance increases. As a result
the net force pulling down on the object becomes less until it equals zero,
resulting in no acceleration and a **terminal velocity** for the remainder
of the fall. As a rule, if two objects are dropped at the same time in
a non-freefall situation, the first one to reach its terminal velocity
will reach the ground last.

**Momentum**

** Momentum** is simply inertia in motion. Inertia is measured
with mass and motion is measured with velocity. To determine momentum we
simply combine the mass and velocity:

Momentum is much like inertia, it tells us the tendency of an object to continue in its current state of motion. Just as we did not stop at identifying inertia, but rather explored its relationship with the state of motion and forces, we must look at the effect of momentum to gain more information about the behavior of objects in motion.

The change in momentum is called ** impulse**. This relationship
can be found by exploring the acceleration of an object. If an object's
momentum changes either the mass or velocity must change:

Another important concept is the ** conservation of momentum**.
This simply means that the momentum of a