00:03
Starting with what we know about this problem, we know that the rock starts 250 meters up and the tourist is at zero meters on the ground.
00:11
The rock has an initial velocity of zero meters per second and it's accelerating with gravity at negative 9 .8 meters per second squared.
00:17
So to figure out how fast this rock is going when it hits the ground, we take the final velocity squared equals initial velocity squared plus two times acceleration times changing position equation.
00:29
And we make sure that we have the right signs when we plug the numbers in.
00:33
So acceleration has to be negative, and the change in position is also negative, because this rock ends at zero and starts at 250.
00:42
So it's always final minus initial.
00:44
So when you do out the map, you find that the final velocity is 70 meters per second.
00:50
Now, with this type of equation, we have to take the square root of a number to find an answer.
00:55
That means the answer will not tell you direction.
00:59
Because that's the nature of taking the square root of something.
01:02
When you take the square root, the answer is either positive or negative.
01:06
So once we get that answer of 70 meters per second, we have to think about the direction of travel of this rock, which is down toward the ground in the negative direction.
01:14
So we add the negative sign after the fact...