Question

The diameter of the driving wheel of a bus is 140 cm. How many revolutions per minute must the wheel make in order to keep a speed of 66 km per hour?

Answer 1

We have:
Diameter of the wheel = 140 cm, desired speed of the bus = 66 km/hr
∴ Distance covered in 1 revolution = The circumference of the wheel = $\mathrm{\pi d}$ = $\left(\frac{22}{7}\times 140\right)\mathrm{cm}=440\mathrm{cm}$.
Now, the desired speed of the bus = 66 km/hr =$\frac{66\times 1000\times 100}{60}=1,10,000\mathrm{cm}/\min$.
∴ Number of revolutions per minute = $\frac{110000}{440}=\frac{1000}{4}=250$.

Thus, the bus must make 250 revolutions per minute to keep the speed at 66 km/hr.