Final answer:
At the top of its trajectory, a projectile fired straight upward at 143 m/s will have a speed of 0 m/s because gravity will have decelerated it to a stop in the vertical direction.
Step-by-step explanation:
The student has asked how fast a projectile is moving at the instant it reaches the top of its trajectory. In this case, the projectile is fired straight upward at 143 m/s. When a projectile reaches the top of its trajectory, its vertical velocity is 0 m/s because gravity has decelerated it to a stop before it starts to fall back down. So, at the top of its trajectory, the projectile is momentarily stationary in the vertical direction. However, if there were any horizontal motion (which is not the case here), that motion would continue unaffected since gravity only affects the vertical component of the velocity.
No horizontal velocity is indicated in this problem, so the speed at the top of the trajectory is 0 m/s.