00:01
Okay, we're trying to find the force, p, that pulls upward, as well as the tensions, a, b, and c.
00:10
I've labeled these masses 1, 2, 3, and 4, with 4 being the lightest and 1 being the heaviest.
00:21
Okay, so how are we going to go about doing this? but one way you can do these problems when you have a bunch of coupled objects like this, you can treat your system as we draw a little dashed line here, where that dashed line represents our system.
00:44
So because that's our system, we could say that the sum of the forces equals the mass of the system times this acceleration.
00:54
Everything has the same acceleration in this system.
00:58
So we could say, well, what's the force acting on this system? well, the only forces are the force up and the total mass times the gravitational acceleration down.
01:12
So in other words, the force up minus the weight down equals the total mass times its acceleration.
01:19
So that's one way to think about it is think of it as one big block instead of a bunch of small pieces or the other way is to look at each little piece individually and then eventually solve for what the external force p is.
01:38
Both ways are just perfectly valid but i think we're going to use the...
01:50
Let's look at each one individually.
01:52
I think that often makes the most sense to people but i just wanted to discuss the other way which is maybe a little bit easier okay so and if we do it this way we'll be able to solve part b sort of simultaneously so let's draw free body diagrams of all of our blobs here we'll do it in green so starting with the first one its weight is pulling it down and the tension is pulling it up the second one, tension from a is pulling it down, and its own weight is pulling it down, and the tension from b is pulling it up.
02:44
The third one, the tension from b and its own weight are pulling it down, the tension from c is pulling it up, the fourth one, eagle from c is pulling it down, and its own weight is also pulling it down.
02:58
And pulling it up is the force p...