A projectile is fired straight upward at 141m/s. How fast is it moving at the instant it reaches the top of its trajectory?

For the object launched straight up, the velocity at the top of the path would have been 0m/s.

Suppose it is launched upward at 45 degrees above the horizontal plane, it would have been 141m/s cos 45⁰ = 99.7 m/s.

A projectile is an object upon which the only force acting is gravity. Many projectiles not only undergo a vertical motion, but also undergo a horizontal motion. That is, as they move upward or downward they are also moving horizontally.

There are the two components of the projectile's motion - horizontal and vertical motion. And since perpendicular components of motion are independent of each other, these two components of motion can (and must) be discussed separately.

Conceptual notions about projectiles:

A projectile is any object upon which the only force is gravity,

Projectiles travel with a parabolic trajectory due to the influence of gravity,

There are no horizontal forces acting upon projectiles and thus no horizontal acceleration,

The horizontal velocity of a projectile is constant (a never changing in value),

There is a vertical acceleration caused by gravity; its value is 9.8 m/s/s, down,

The vertical velocity of a projectile changes by 9.8 m/s each second,

The horizontal motion of a projectile is independent of its vertical motion.

To learn more, see links below:

Describing Projectiles With Numbers: (Horizontal and Vertical Velocity)

Characteristics of a Projectile's Trajectory