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Speed simply tells us how fast an object can move.

*A moving car*

Speed is defined as the distance travelled by an object in unit time. The SI unit of speed is \(metre\ per\ second\). This is represented by the symbol \(m\ s^{ –1}\) (or) \(m/s\).

**Other units of speed**:

\(centimetre\ per\ second\) (\(cm\ s^{–1}\))

\(kilometre\ per\ hour\) (\(km\ h^{–1}\))

Speed is the scalar quantity, which only tells the magnitude (numerical value) of how much speed an object travels.

Speed or average speed is the total distance covered divided by the total time taken. Thus,

\(\text{Speed} = \frac{\text{Total distance travelled}}{\text{Total time taken}}\)

The speed of an object can be determined by measuring the time taken by it to cover a certain distance. Based on the speed, the moving object may have a uniform or non-uniform motion.

Actual speed is the distance travelled by the body at a particular time.

A car moving at a constant speed covers a distance of \(100\) \(kilometres\) in one hour, implying that it had travelled at a speed of \(100\) \(kilometres\ per\ hour\). This shows that the total distance covered by the car in one hour is \(100\) \(kilometres\). The car does not have to travel at a constant speed because it is the average speed of the car that is calculated. The term 'average speed' can be used to determine 'speed'.

If an object travels a distance \(s\) in time \(t\), then its speed \(v\) can be written mathematically as,

\(\text{v} = \frac{\text{s}}{\text{t}}\)

**Example**:

Measure the time it takes you to walk from your house to your bus stop or the school. For example, if you consider that your average walking speed is \(5\ km/h\), estimate the distance of the bus stop or school from your house.

**Solution**:

In this activity, we need to calculate the time by using the average speed value.

Speed is the distance travelled by an object in a unit of time. Here, the unit is given in \(km/hr\), so we need to calculate walking time in an hour. For example, if the person takes \(12\ minutes\) to reach school, then the time in hours can be calculated as,

Time \(=\) $\frac{12}{60}$ \(=\) 0.2 \(h\)

$\begin{array}{l}\mathit{Speed}\phantom{\rule{0.147em}{0ex}}(v)=\phantom{\rule{0.147em}{0ex}}\frac{\mathit{Distance}\phantom{\rule{0.147em}{0ex}}(s)}{\mathit{Time}\phantom{\rule{0.147em}{0ex}}(t)}\\ \mathit{Distance}\phantom{\rule{0.147em}{0ex}}(s)\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\mathit{Speed}\phantom{\rule{0.147em}{0ex}}(v)\times \phantom{\rule{0.147em}{0ex}}\mathit{Time}\phantom{\rule{0.147em}{0ex}}(t)\end{array}$

Distance \(=\) $5\xb70.2$ \(=\) 1 \(km\)

In terms of \(metre\), the value should be multiplied by \(1000\).

\( 1 × 1000\) \(=\) 1000 \(m\)

We can also convert \(km/hr\) to \(m/min\). In every case, the answer will be the same. The only thing to consider is that all the quantities should be in the same unit. In this case, we should prefer \(m/min\) for a shorter distance or short walk time.