Aball is thrown with a velocity of 35 meters per second at an angle of 30° above the horizontal. which quantity has a magnitude of zero when the ball is at the highest point in its trajectory?

The quantity that has a magnitude of zero when the ball is at the highest point in its trajectory is the vertical velocity.

In fact, the motion of the ball consists of two separate motions:
- the horizontal motion, on the x-axis, which is a uniform motion with constant velocity , where 
- the vertical motion, on the y-axis, which is a uniformly accelerated motion with constant acceleration  directed downwards, and with initial velocity . Due to the presence of the acceleration g on the vertical direction (pointing in the opposite direction of the initial vertical velocity), the vertical velocity of the ball decreases as it goes higher, up to a point where it becomes zero and it reverses its direction: when the vertical velocity becomes zero, the ball has reached its maximum height. 

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