A projectile is fired horizontally with an initial speed of 50.0 m/s. Neglect air resistance.
-What is the speed of the projectile 3.00 s after it is fired?
A)29.4 m/s
B)50.0 m/s
C)58.0 m/s
D)79.4 m/s
E)98.6 m/s

Question

Quizplus · Accepted Answer

Since the projectile is fired horizontally, its initial velocity in the y-direction is zero. 
Thus, the velocity of the projectile after 3.00 seconds is entirely due to its constant acceleration in the downward direction (since neglecting air resistance, the only force acting on the projectile is gravity). 
Using the kinematic equation v = v0 + at, where v0 is the initial velocity, a is the acceleration, and t is the time: 
v = 0 + (9.81 m/s^2)(3.00 s) = 29.4 m/s 
However, this velocity is only in the vertical direction. Since there is no acceleration in the horizontal direction, the horizontal velocity remains constant at 50.0 m/s. 
Thus, the resultant velocity of the projectile is given by the Pythagorean theorem: 
vresultant = sqrt((50.0 m/s)^2 + (29.4 m/s)^2) = 58.0 m/s, 
which is answer choice C.

A Projectile Is Fired Horizontally with an Initial Speed of 50.0