Kinematics - Lesson 1

Two branches in physics examine the motion of objects:
• Kinematics: describes the motion of objects, without looking at the cause of the motion
(kinematics).
• Dynamics: relates the motion of objects to the forces which cause them (dynamics).

As we work through these two units on kinematics and dynamics (and
through the rest of physics) we will discuss two kinds of measurements
(quantities):
• Scalar: scalars have magnitude (a number value), but no
direction.
Examples: time, mass, distance. Mass is a great example, since it
has a number value (like 58 kg), but we don't give it a direction
(like “East”).
• Vector: have magnitude and direction
Examples: velocity, force, displacement. Force has a magnitude (like 37 N) and a direction
(like "pushed to the left").

Distance & Displacement
In kinematics we need to be able to have a way to describe the motion of the objects we will be
studying, whether it's a car or an atom.
• The most basic information you must have to describe the motion of an object is its
displacement, and the time it took to move that far.
• The displacement of an object is always measured from some reference point (which is usually “zero”, at a location at the start of the motion of the object).
• Although we use the words “distance” and “displacement” interchangeably in
everyday language, they mean very different things in physics.
• The distance between two objects is scalar, since it doesn't matter which
direction you measure it from. e.g. “We are standing 2.3m apart.”
• The displacement of an object is a vector, since you have to state the direction
the object has traveled. e.g. “The car moved 2.56km east.”

The most simple formula for calculating the displacement of an object is…

Δd = df - di

• The Δ symbol is the greek letter “delta” and means “a change in…”
• The subscript “f” and “i” stand for final and initial.
• So, in this formula, we calculate the displacement of an object by taking the final position

minus the initial position.

Example 1: A car is passing a mark on the road that says
600m, and then passes another one 10 seconds later that says
1000m. Determine the distance the truck moved.

Δd = df - di = 1000 - 600 = 400m

Note: If the example had asked for the displacement, we would have to include a direction (like “east”) in our answer, but in this case as no direction is mentioned, we needn't mention any direction. 

To recap, Distance, is just one thing, one magnitude with no direction (a scalar), Displacement has a magnitude and a direction.