ball is thrown straight up with an initial speed of $30 \mathrm{~m} / \mathrm{s}$. What is its speed after $4.2 \mathrm{~s}$ ?
A) $42 \mathrm{~m} / \mathrm{s}$
B) $72 \mathrm{~m} / \mathrm{s}$
C) $30 \mathrm{~m} / \mathrm{s}$
D) $11 \mathrm{~m} / \mathrm{s}$

Question

Quizplus · Accepted Answer

To find the speed of the ball after 4.2 seconds, we use the equation $v = u + at$, where $v$ is the final velocity, $u$ is the initial velocity (30 m/s upwards), $a$ is the acceleration due to gravity (-9.8 m/s$^2$ downwards), and $t$ is the time (4.2 s). Plugging in the values, we get $v = 30 + (-9.8)(4.2) = 30 - 41.16 = -11.16 \mathrm{~m/s}$. The negative sign indicates the direction is opposite to the initial throw direction, but since the question asks for speed, we consider the magnitude, which is approximately 11 m/s.

Ball Is Thrown Straight Up with an Initial Speed Of 30