00:01
From what height would you have to drop a car for its impact to be equivalent to a 20 -mile -per -hour collision? well, i'm going to write down that the kinetic energy plus the potential energy initial, that means before you drop it, equals the kinetic energy plus the potential energy final, that means at the impact.
00:39
Now, initially, the car is at rest, so it has zero kinetic energy.
00:50
At the end, the car has hit the ground, so it has zero potential energy.
00:58
So the initial potential energy is the mass times gravity times the height.
01:07
And the kinetic energy is one -half times the mass.
01:12
Times the speed to the second power.
01:16
Notice that mass cancels out.
01:20
We're asked to figure out the height, so i'm going to divide by g on both sides.
01:30
And so the height is one -half v squared over g.
01:44
All right.
01:45
So i'm going to try to figure out where i can write this.
01:49
I think i'll just write it right next here.
01:53
So the height is one -half v squared over g.
01:59
Now, g is 9 .81 meters per second squared.
02:09
V i'm going to put in here, and it's going to square.
02:13
But i notice that v is 20 miles.
02:25
Per hour.
02:27
And i do not like to work with standard units.
02:31
I like to work with si units.
02:36
So i'm going to convert this...