Speed and acceleration
When we talk about speedThe distance travelled in a fixed time period, usually one second., we could be talking about two different things:
- average speed – the speed of an object measured over the whole journey
- instantaneous speed – the speed of an object at the very instant of being measured
average speedAverage speed = distance / time, over a known distance and time. For example, the average speed of the bus between Edinburgh and Glasgow was 30 m s-1. is defined by this mathematical relationship:
\(average\,speed = \frac{{total\,distance\,travelled}}{{total\,time\,taken}}\)
Average speed is measured in metres per second \(m\, s^{-1}\).
Calculating average speed
Instantaneous speed is the speed of an object at a particular moment in time. Instantaneous speed is measured in metres per second \(m\, s^{-1}\).
The instantaneous speed of an object can be calculated if we know the distanceNumerical description of how far apart two things are. For example, the distance from Edinburgh to Glasgow is approximately 50 miles. travelled in a very short period of time.
We might think about the speedometer on a car or most kinds of speed cameras.
Question
A speed camera flashes twice at a 0.7 second interval. If a car travels \(4.2 m\) in this time, what is its instantaneous speed?
\(v = \frac{d}{t}\)
\(= \frac{{4.2}}{{0.7}}\)
\(= 6.0\, m\, s^{-1}\)
An object is travelling at a steady or constant speedWhen the speed of body does not change, eg the car was travelling at a constant speed of 30 m s-1. when its instantaneous speed has the same value throughout its journey. For example, if a car is travelling at a constant speed the reading on the car's speedometer does not change. The speedometer reads the speed of the car at each moment in timeTerm that describes the order and duration of events. For example, the Physics lesson was 50 minutes long. throughout the car's journey. In cases like this, where the motion involves constant speed, the instantaneous speed of the object can be worked out using the relationship:
\(speed = \frac{distance}{time}\)
