```
v = u + gt
```
where:
* v is the final velocity of the rock (in meters per second)
* u is the initial velocity of the rock (in meters per second)
* g is the acceleration due to gravity (approximately 9.8 meters per second squared)
* t is the time taken by the rock to fall (in seconds)
In this case, the rock is initially dropped from rest so u equals to zero:
```
v = u + gt
v = 0 + 9.8 * 5
v = 49 meters per second
```
Therefore, the speed of the rock after falling for 5 seconds would be approximately 49 meters per second, not 100.
