00:01
For this problem, we are told that andrea, a 63 -kilogram sprinter starts a race with an acceleration of 4 .2 meters per second squared.
00:08
We're asked, what is the net external force on her? so this is solved using newton's second law.
00:15
Force equals mass times acceleration.
00:18
In fact, we just need to plug in the numbers directly into that equation without needing to do any rearranging.
00:25
So the mass is 63 .0 kilograms.
00:28
The acceleration is 4 .2 meters per second squared.
00:34
So, 63 times 4 .2 becomes 264 .6 kilogram meters per second squared, which is the same thing as 264 .6 newtons.
00:51
Then for part 25, we're asked if the sprinter from the previous problem accelerates at that rate for 20 meters, and then maintains that velocity for the remainder of a 100 meter dash, asked what will her time be for the race? so for answering this, we'll need to consider our kinematic equations.
01:11
So i'm just going to bring these up on screen briefly.
01:16
So we'll need to consider this in sort of two different phases.
01:22
So the first phase will be the amount of time, oops, let's see here, the first phase will be the amount of time that it takes for or to traverse those 20 meters.
01:35
So we know that the initial velocity would be zero, and then we want to figure out, again, as i said, the amount of time it takes to go 20 meters with that level of acceleration.
01:47
So we want to use equation three there.
01:52
So in this case, we would have that, oh, actually, let me correct myself here.
01:58
No, never mind.
01:59
That is what we want.
01:59
So we'll be using equation 3 here.
02:02
We'd have that delta x, which is 20 meters, must be equal to one half of the acceleration, 4 .2 meters per second squared, times t squared...