Suppose the two photographs (shown in figure 1 and figure 2), had been taken at an interval of 10 seconds. If a distance of 100 metres is shown by 1 cm in these photographs, calculate the speed of the blue car (indicated using arrows).

Given,

The distance covered, ${\mathrm{d}}_{\mathrm{c}} =100 \mathrm{m}$

Time interval, $\mathrm{t} = 10 \mathrm{s}$

Let the distance measured by scale, ${\mathrm{d}}_{\mathrm{s}} = 2 \mathrm{cm}$

Taken,

$100 \mathrm{m} = 1 \mathrm{cm}$

∴ Actual distance covered by the blue car (as per measured), 

$\mathrm{d} = {\mathrm{d}}_{\mathrm{c}}\times {\mathrm{d}}_{\mathrm{s}}$

$\mathrm{d} = 100 \times 2$

$\mathrm{d} = 200 \mathrm{m}$

Now the speed of the blue car,

$\mathrm{s} =\frac{\mathrm{d}}{\mathrm{t}}$

$\mathrm{s} = \frac{200}{10}$

$\mathrm{s} = 20 \mathrm{m}$

Thus the speed of the blue car, $\mathrm{s} = 20 \mathrm{m}$