Ncert Exercises & Solutions

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

Hint: Apply the concept of relative motion.

Step 1: Sketch the diagram

Consider the diagram below:

Step 2: Resolve the velocity of the ball in horizontal and vertical directions.

The boy throws the ball at an angle of 60 $°$ .
$∴$ Horizontal component of velocity = 4 cos $θ$
$= (10 m / s) \cos 60^{\circ} = 10 \times \frac{1}{2} = 5 m / s$
Speed of the car = 18 km/h = 5 m/s.

Step 3: Apply relative velocity

As the horizontal speed of the ball and car is the same, hence the relative velocity of the car and ball in the horizontal direction will be zero.

Only vertical motion of the ball will be seen by the boy in the car, as shown in fig. (b)

Step 4: Conclusion

As the horizontal speed of the ball and car is the same, hence the relative velocity of the car and ball in the horizontal direction will be zero.

Only vertical motion of the ball will be seen by the boy in the car, as shown in fig. (b)

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