$A$ boy throws a ball in the air at $60^{\circ}$ to the horizontal,along a road with a speed of $10 \, m/s$. Another boy sitting in a passing car observes the ball. Sketch the motion of the ball as observed by the boy in the car,if the car has a speed of $18 \, km/h$. Give an explanation to support your diagram.

(N/A) The boy throws the ball at an angle of $60^{\circ}$ with a speed of $10 \, m/s$.
The horizontal component of the ball's velocity is $v_x = v \cos \theta = 10 \cos 60^{\circ} = 10 \times 0.5 = 5 \, m/s$.
The speed of the car is $18 \, km/h = 18 \times \frac{5}{18} = 5 \, m/s$.
Since the horizontal component of the ball's velocity $(5 \, m/s)$ is equal to the speed of the car $(5 \, m/s)$,the relative horizontal velocity of the ball with respect to the car is $v_{rel,x} = 5 - 5 = 0 \, m/s$.
Therefore,the boy in the car will observe the ball moving only in the vertical direction,which appears as a straight vertical line path.