Useful terms for describing motion
The following terms are useful for describing and calculating different types of motion:
- average speed
- The speed at a particular point in time, eg the speed of the car at the corner was 5ms-1. (normally referred to as 'speed')
- acceleration
Average speed
Average speed is defined by the mathematical relationship shown below.
\(average\,speed = \frac{{total\,distance\,travelled}}{{total\,time\,taken}}\)
Average speed is measured in metres per second (\(m\,s^{-1}\)).
In the classroom, average speed can be investigated for a trolley travelling down a ramp.
This is carried out using two light gates that are a known distance apart. Watch this video to see the correct procedure for this investigation.
The video below shows a real life example of calculating average speed.
Calculating the speed of a car
Instantaneous speed
Instantaneous speed is the speed of an object at a particular moment in time. It is measured in metres per second (\(m\,s^{-1}\)), ie not over a long distance or long time period. Usually in the lab, instantaneous speeds are measured over a distance of a few centimetres in time of tenths of a second.
To do this in the classroom, a short piece of card is usually attached to a vehicle (Numerical description of how far apart two things are. For example, the distance from Edinburgh to Glasgow is approximately 50 miles. = width of card) and the time is measured using a computer and light gate (time = time taken for card to pass through the light gate).
\(instantaneous\,speed = distance \div time\)
\(= (width\,of\,card \div time\,taken\,to\,pass\,through\,light\,gate)\)
You can see the correct procedure for carrying out this experiment in the video below.
