We can solve this using the first equation of motion:

$v = u + at$

Where:

• $v$ is the final velocity,
• $u$ is the initial velocity (which is 0, since the particle starts from rest),
• $a$ is the acceleration (given as $9 \, \text{m/s}^2$),
• $t$ is the time (given as 4 seconds).

Now, plug in the values:

$v = 0 + (9 \, \text{m/s}^2) \times (4 \, \text{sec}) = 36 \, \text{m/s}$

Thus, the velocity after 4 seconds is 36 m/s.

Do you have any further questions or want more details?

Here are 5 related questions for practice:

1. What is the velocity after 6 seconds if the acceleration is 9 m/s²?
2. If the particle had an initial velocity of 5 m/s, what would its velocity be after 4 seconds?
3. How far does the particle travel in the first 4 seconds?
4. What is the velocity if the particle starts from rest and accelerates at 12 m/s² for 5 seconds?
5. How long does it take for the particle to reach a velocity of 50 m/s with an acceleration of 9 m/s²?

Tip: When dealing with constant acceleration problems, always choose the right kinematic equation based on the known variables.