00:01
So for this question you've sent in, it's the last part of the question, and it says if the stone is thrown downwards with a velocity of 8 meters per second, how long does it take to reach the ground? well, so we're going to use our cvat equations again.
00:14
We know that s is 400 meters.
00:16
We know that u is, our initial velocity is 8 meters per second.
00:23
We don't care about our final velocity v.
00:26
Our acceleration is 9 .8 meters per second squared, and our time is what we're interested in.
00:31
So we're going to use the suva equation, s is equal to a ut plus a half 80 squared.
00:38
So 400 is equal to 8t plus a half times by, a half times by 9 .8 is 4 .9t squared.
00:53
So therefore we're going to get 4 .9t squared plus 80 minus 400.
01:04
So then we're just going to use our quadratic formula to work out the solutions to this.
01:08
So our quadratic formula is minus b plus or minus a square root of b squared minus 4ac over 2a, which is where our b is 8, our a is 4 .9, and our c is minus 400.
01:27
So we're going to get minus 8 plus or minus a square root of 8 squared minus 4 times minus 4, sorry, not times, sorry, minus, times 4 .9 times minus 400 over 2 times a, which is 2 times 4 .9.
01:49
So we're going to get minus 8, if we put the semi -calculator, minus 8 plus square of 8 squared minus 4 times 4 .9 times minus 400 over 2 times 4 .9 .9.
02:09
We're going to get two values for t.
02:12
It's t is equal to 0 .8 .3 seconds and t is equal to 0 .98 989 seconds or just 9 .9 seconds.
02:34
Obviously we cannot have a negative time...