00:01
We have a particle of mass m moving along a curve traced out by the vector r of t.
00:06
We're asked to find the force acting at the particle at time t.
00:10
Well, force is a vector, and force is mass times acceleration.
00:16
So we can find our acceleration vector by taking two derivatives of our position vector.
00:23
So our first derivative will be velocity, and that will give us a three.
00:27
And remember, when you're doing trig functions, you're going to take the derivative of the outside.
00:33
So cosine becomes a negative sign, so the negatives cancel out.
00:37
You're going to write the 3t, and then you're going to multiply by the derivative of the inside.
00:41
That's why you get that extra 3...