00:01
So in this problem, we have a spring with a 10 kilogram mass on it, and initially it stretches 9 .8 centimeters.
00:06
So we can kind of think about this right.
00:08
Here's a spring that's got a 10 kilogram mass hanging on it here.
00:17
Initially, it stretches when you hang the mass on there.
00:20
Then at time t equals zero, there's some force applied to it, given by f of t is 140 cosine 10t, and forces in newton's, and of course time t is at 7.
00:31
Seconds.
00:33
We're first asked to find the spring constant k.
00:36
Well, initially when you put that weight on there, it stretches and stops, right? so i have k times y, the displacement that it stretches is equal to mass times gravity.
00:50
Well, this displacement right here is 0 .098 meters, like there's 100 centimeters in a meter.
01:00
So this means i have k times 0 .098.
01:04
Is equal to my mass, 10 kilograms, and g, our gravitational pull, is 9 .8 newton's per meter squared, or so per, not meter squared, sorry, nons per kilogram.
01:25
All right, so that means that k is 10 times 9 .8 over kilograms cancel, over 0 .098.
01:36
And so this is 98 over 0 .098, which is 1 ,000 newtons per meter.
01:50
It is my spring gravitational constant.
01:55
My spring constant.
01:57
Okay.
01:59
So, next we're asked to write the initial value problem with conditions.
02:22
Okay.
02:24
Well, if we consider that force is mass times acceleration.
02:31
We have f of t is equal to this 140 cosine 10t, 140 cosine of 10t, minus ky, right, my spring constant times the displacement on it.
02:58
Okay, so that's the sum of the forces.
03:00
And that's equal to mass times acceleration.
03:02
Well, acceleration is the second derivative of displacement with respect to time, which i could write this as m y double prime.
03:12
And we know k is a thousand.
03:14
So that means i have 140 cosine of 10 t is equal to my mass is 10, 10 y double prime, plus a thousand y.
03:39
I can divide everything here by 10...