A ball is launched upward from a baseball field.Assuming that it begins with an upward velocity of 10.5 m/s,what is its speed after three seconds have elapsed? (The acceleration due to gravity on Earth is 9.8 m/s².)
A) 0.7 m/s
B) 10.5 m/s
C) 18.9 m/s
D) 39.9 m/s

Question

Quizplus · Accepted Answer

Using the formula

v = u + at

where

v = final velocity 
u = initial velocity = 10.5 m/s (given)
a = acceleration due to gravity = -9.8 m/s² (negative because it opposes the upward motion)
t = time = 3 s (given)

Substituting the values, we get

v = 10.5 - 9.8(3) 
v = 10.5 - 29.4 
v = -18.9 m/s

Note that the negative sign indicates that the velocity is downward. However, the problem asks for the speed, which is the magnitude of the velocity. Therefore, we take the absolute value, giving

speed = |-18.9| 
speed = 18.9 m/s

Thus, the ball's speed after three seconds is 18.9 m/s, which corresponds to choice C.

A Ball Is Launched Upward from a Baseball Field