Lesson 7: Motion in a Straight Line


Two branches in physics examine the motion of objects:

  1. Kinematics: describes the motion of objects, without looking at the cause of the motion (this is what we will be focusing on now)
  2. Dynamics: relates the motion of objects to the forces which cause them (dynamics pops up later on in Physics 20).
"Quantity" is the root word for "quantitative" measurements. This means you're supposed to get a number answer. A qualitative measurement describes qualities of the data.

In kinematics and dynamics we will discuss two kinds of measurements (quantities):

  1. scalar: scalars have magnitude (a number value), but no direction
    e.g. 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 "down").
  2. vector: have magnitude and direction
    e.g. velocity, force, displacement. Force has a number value (like 37 N) and a direction (like "pushed to the left").

Position & Time

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 position and the time it was at that position.

The most simple formula for calculating the change in an object's position is…

Δd = df - di

Notice that the word "determine" has been bolded in the question. This is a "directing word" telling you what to do in the question. Get used to seeing these, since they are used on the Physics 30 diploma.

Example 1: A truck is passing a mark on the road that says 300m, and then passes another one 10s later that says 450m. Determine the distance the truck moved.

Δd = df - di = 450 - 300 = 150m

Note: If the example had asked for the displacement, we would have to include a direction (like "east") in our answer.

Average Velocity

This leads to the first major formula for the calculation of "average velocity".

v = velocity (m/s)
Δd = displacement (m)
Δt = time (s)

Example 2: I try to run the 100m race to break the world's record! Unfortunately, it takes me 16.83s to complete the run. Determine my average velocity.

v = d / t

= 100m / 16.83s

v = 5.94m/s

Note that in the above example, the displacement and the velocity were positive numbers.

Example 3: A high speed train might be built between Edmonton and Calgary. It could travel between the two cities at an average velocity of +227km/h. The trip would take 1.2h. Determine how far apart Edmonton and Calgary are.

v = d / t

d = vt

= (227km/h) (1.2h)

= 272.4km = 2.7e2 km

Uniform Motion

If the velocity of an object is the same throughout the entire time, it has a constant (AKA uniform) velocity.

Instantaneous Velocity

But what about when you are calculating the velocity of an object traveling without uniform velocity?

Velocity vs Speed

One last note for you. Avoid using the word "speed" when describing any velocity.

"What speed did you drive today along Yellowhead?" -> "I drove at 72km/h."

"What was your velocity along Yellowhead?" -> "My velocity was 72km/h East."