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Question
A boy throws a ball in air at 60° to the horizontal along a road with a speed of 10 m/s (36 km/h). Another boy sitting in a passing by car observes the ball. Sketch the motion of the ball as observed by the boy in the car, if car has a speed of (18 km/h). Give explanation to support your diagram.
Solution
The situation is shown in the below diagram.
According to the problem, the boy standing on the ground throws the ball at an angle of 60° horizontal at a speed of 10 m/s.
∴ The horizontal component of velocity, ux = 10 cos θ
ux = (10 m/s) cos 60° = `10 xx 1/2` = 5 m/s
The vertical component of velocity, uy = 10 sin θ
uy = (10 m/s) sin 60° = `10 xx sqrt(3)/2 = 5sqrt(3)` m/s
Speed of the car = 18 km/h = 5 m/s
As the horizontal speed of ball and car is same, hence the relative velocity of ball w.r.t car in the horizontal direction will be zero.
Only the vertical motion of the ball will be observed by the boy in the car, as shown in the above diagram.
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