If a ball is dropped from rest, what is it’s velocity after 4 seconds

-39.2 m/s directed downwards

Explanation:

We can solve this problem with the following equation:

Where:

is the ball's velocity at 4 s

is the ball's intial velocity, since it was dropped from rest

is the acceleration due gravity, aways directed downwards

is the time

Solving:

Finally:

The negative sign indicates the velocity is downwards

the device is called “ rocket engine”