For general projectile motion with no air resistance, the magnitude of the vertical component of a projectile’s velocity
A) remains zero.
B) remains a non-zero constant.
C) continuously increases.
D) continuously decreases.
E) first decreases and then increases.
The correct answer and explanation is:
Correct Answer: E) first decreases and then increases
In general projectile motion without air resistance, the vertical component of a projectile’s velocity does not stay constant. Instead, it first decreases to zero, then increases in the opposite direction.
When an object is projected at an angle to the horizontal, its motion can be broken into two components: horizontal and vertical. The horizontal component of velocity remains constant because there is no horizontal force (neglecting air resistance). However, the vertical component is affected by gravity.
At the moment the projectile is launched, it has an upward vertical velocity. As it rises, gravity acts in the opposite direction of this motion. This causes the vertical speed to decrease steadily at a rate of approximately 9.8 m/s². Eventually, at the peak or maximum height of its flight, the vertical velocity reaches zero.
After reaching the peak, the projectile begins to fall. Now, gravity continues to act downward, but this time it is in the same direction as the motion. As a result, the magnitude of the vertical velocity begins to increase again, but now the velocity is directed downward.
Thus, the magnitude (size) of the vertical velocity first decreases as the projectile goes up, then increases as it comes down. This behavior occurs in every projectile motion problem where there is an initial vertical component and gravity is the only vertical force.
This pattern explains why the correct answer is E: first decreases and then increases. The key idea is that gravity changes the vertical velocity over time, but it does so in a smooth and predictable way.