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(problem reference 2 - 1)
a tennis ball is shot vertically upward with an initial speed of 20.0 m/s. what is the velocity of the ball, 3.00 seconds after it was launched?
9.40 m/s, upward
38.8 m/s, upward
38.8 m/s, downward
9.40 m/s, downward
29.4 m/s, downward

Explanation:

Step1: Identify the kinematic - equation

The kinematic equation for velocity $v = v_0+at$. In vertical - motion near the surface of the Earth, $a=-g=- 9.8\ m/s^2$ (negative because the acceleration due to gravity acts downward), $v_0 = 20.0\ m/s$ (initial upward velocity), and $t = 3.00\ s$.

Step2: Substitute the values

Substitute the values into the equation: $v=v_0+at=20+( - 9.8)\times3$.
First, calculate $( - 9.8)\times3=-29.4$. Then $v = 20-29.4=-9.4\ m/s$. The negative sign indicates the direction is downward.

Answer:

9.40 m/s, downward