## 5 thoughts on “3.39”

1. Jesse Travis says:

Should you always guess which way an object is accelerating, or moving, and define your positive coordinate in that direction?

1. tutorpaul says:

Yes, unless the problem statement defines positive directions of motion we must choose a direction, and we are free to choose it however we like. In my videos I will always choose consistent directions (i.e. all masses are able to move in their positive directions simultaneously, and thus all my kinematic constraints come out with positive coefficients.

If you choose a different coordinate your EOM may look different, but it will be equivalent if you have performed the analysis correctly.

2. Farris04 says:

When deriving the equilibrium EOM, (12:15), I do not see how $-m_1g\sin(\alpha)$ on the left hand side cancels out the $-2m_1g\sin(\alpha)$ on the right hand side. Am I missing something?

1. tutorpaul says:

Good catch! I made an error, if you look at the work in red you'll see that I forgot to write a 2 on the second line. Where I wrote

it should have been

1. pacofalcon says:

Paul you are awesome sir. I would not make it through 363 without your help.