A tennis ball is thrown vertically with an initial speed of 20.0 m/s at time t = 0 s. Approximately, what is the initial speed of the ball relative to an observer in a car that moves horizontally past the launch location with a constant speed of 30 m/s?
A)10 m/s
B)20 m/s
C)30 m/s
D)36 m/s
E)50 m/s

Question

Quizplus · Accepted Answer

The initial speed of the ball relative to the observer is the vector sum of the initial speed of the ball and the speed of the car. The vector sum of two perpendicular vectors is the square root of the sum of the squares of the two speeds. So, the initial speed of the ball relative to the observer is sqrt(20^2 + 30^2) = 36 m/s.

A Tennis Ball Is Thrown Vertically with an Initial Speed