Science Vault - Year 11 HSC Physics

8.4 - Moving About

8.4.1 - Kinematics (motion in a straight line)

definitions / instantaneous and average velocity


Some important definitions are shown in the table below:

Quantity Symbol Definition Example
SI Unit
Vector or Scalar
s or r
The distance moved in a particular direction. The displacement from London to Rome is 1.43 x 106 m southeast.
v or u

The rate or change of displacement

The average velocity during a flight from London to Rome is 160 ms-1 southeast.
v or u

The rate of change of distance.

The average speed during a flight from London to Rome is 160 ms-1 .
The rate of change of velocity.

The average acceleration of a plane on the runway during take-off is 3.5 ms-2 in a forwards direction. This means that on average, its velocity changes every second by 3.5 ms-1 .

These technical terms should not be confused with their everyday use. In particular you should note that:

  • vector quantities always have a direction associated with them.

  • generally, velocity and speed are NOT the same thing. This is particularly important if the object is not going in a straight line.

back to top

Instantaneous and Average Velocity

The average value of velocity over a period of time is very different to the instantaneous value at a particular instant in time.

Imagine a sprinter during a 100 m race where the sprinter covered the distance in 11.3 seconds. The average speed over the whole race is easy to work out. It is the total distance (100 m) divided by the time (11.3 s) giving 8.5 ms-1.

During the race, however, her instantaneous speed would have changed. If at the end of 2.0 seconds she had travelled 10.4 m, here average speed for the first two seconds would have been 10.04/2.0 = 5.0 ms-1. During these first two seconds her instantaneous speed was increasing - she was accelerating. If she started at rest and her average speed over the whole 2 s was 5.02 ms-1 then her instantaneous speed at 2 s must be more than this. In fact, the instantaneous speed for this sprinter was 9.23 ms-1 at 2 s, but it would have been impossible to work this out from the information given.

You can use the following website to see an animation showing the difference between average speed and instantaneous speed.

back to top

download worksheet 8.4.1.A - kinematics

download experiment 1 - measuring speed


Internet References

  • Under constructtion.

Syllabus References

These references relate to the content covered on this page and can be found in Section 8.4.1 of the syllabus.

1. Vehicles do not typically tavel at a constant speed.


  • Identify that a typical journey involves speed changes.

  • Distinguish between the instantaneous and average speed of vehicles and other bodies.

  • Distinguish between scalar and vector quantities in equations.

  • Compare instantaneous and average speed with instantaneous and average velocity.

  • Define average velocity as:

Students learn to:

  • Plan, choose equipment or resources for, and perform a first-hand investigation to measure the average speed of an object or a vehicle.

  • Solve problems and analyse information using the formula:

    where r = displacement.

Textbook References

Taken from:

Heffernan, D., Parker, A., Pinniger, G. & Harding, J. (2002) Physics Contexts 1, Pearson Education, Melbourne

  • Sections x and x on pp. x - x