00:01
Okay, we were given the small rock is thrown vertically upward with the speed of 22 .0 meters per second from the edge of a roof of a 30 meter tall building.
00:10
And we want to find its speed just before it hits the ground below.
00:13
So let's call up positive y.
00:16
And since this problem involves velocities and distances, we're going to want to use the velocity as a function of distance equation, which is given by the final velocity squared is equal to the initial velocity plus two.
00:31
Times the acceleration times the change in position or in the change in a y position in this case a yf minus y initial now plugging the numbers the final velocity is squared our initial velocity is going to be 22 it's positive since it's aligned with the positive or it's in the same direction as the positive y axis 22 squared and our a here is going to be g and it's going to be negative since it's in the opposite direction of the velocity gravity always points down towards the center of the earth or the center of the mass.
01:15
So it's going to be minus 2 times 9 .81.
01:18
And if we call our y initial 0, then our y final should be negative, meaning that the y final should be negative 30 .0.
01:29
Plugging this all into a calculator, 22 squared is 484.
01:40
2 times 9 .81 times 30 is, it should be a positive because the negative sense can't.
01:50
588 .6 had these two together and we get 172 .6.
02:01
Taking the square root of that and we get the final velocity is going to be negative since it's going down.
02:14
And when we're taking the square root of course, we're always going to get a positive and negative root.
02:19
And you just have to just take the one which makes physical sense...