Question

A boy aims a target at a horizontal distance of 60 m.If the muzzle speed of bullet is 600 m/s, then the height above the target which he should aim is
1) 5 cm
2) 10 cm
3) 15 cm
4) 20 cm

Answer 1

A boy aims a target at a horizontal distance of $60\,\mathrm{m}$. If the muzzle speed of bullet is $600\,\mathrm{m/s}$, then the height above the target which he should aim is 1) $5\,\mathrm{cm}$ 2) $10\,\mathrm{cm}$ 3) $15\,\mathrm{cm}$ 4) $20\,\mathrm{cm}$.

Solution

The acceleration of gravity $g = 10\frac{m}{s^2}$.

The time to cover $60\,\mathrm{m}$

$$t = \frac{s}{v} = \frac{60\,\mathrm{m}}{600\,\mathrm{m/s}} = 0.1\,s.$$

The downward distance moved in this time

$$h = \frac{g t^2}{2} = \frac{10\,\frac{m}{s^2} * (0.1\,s)^2}{2} = 0.05\,m = 5\,\mathrm{cm}.$$

Answer: 1) $5\,\mathrm{cm}$.